In commit b44cbe4 of PR git-lfs#5136 the
"multiplex" argument was added to the FormatArgs() function in our
ssh/ssh.go source file.  Later, the "controlPath" argument was also
added, in commit 448b0c4 of PR git-lfs#5537.

Neither of these arguments is used in the function's body, though,
so we can remove them now.

In commit 448b0c4 of PR git-lfs#5537 our
lfs-ssh-echo test helper utility was updated to examine the value of
any provided ControlMaster command-line argument and simulate the action
of the OpenSSH program when it is passed a ControlMaster argument with
a value of either "yes" or "no".  (The other values OpenSSH supports
for its ControlMaster option are not accepted by our lfs-ssh-echo
utility at the moment.)

When a ControlMaster command-line argument is found, and has the value
"yes", the lfs-ssh-echo program attempts to create a temporary file at
the location given by the ControlPath command-line argument, which must
also be provided.  If the file already exists, or some other error
occurs while creating it, the program halts with a non-zero exit code.

If the ControlMaster argument's value is "no", and the ControlPath
argument is also defined, the program attempts to open the file at
the given path, and exits with a non-zero code if the file does not
already exist or some other error occurs.

This logic is designed to emulate the behaviour of OpenSSH, which
supports the use of ControlMaster and ControlPath arguments to multiplex
connections over a common control socket.  When the ControlMaster argument
is "yes", OpenSSH creates a socket, which may then be shared by other
invocations of OpenSSH by setting ControlMaster to "no" and passing
the socket's path in the ControlPath argument.

Our lfs-ssh-echo program, however, creates its temporary file with
no defined file permissions (i.e., a file mode argument of zero).
This results in a file which can not be opened by any other instances
of the lfs-ssh-echo program.  At the moment this does not cause any
problems with our test suite, because we have no tests which exercise
the SSH version of the Git LFS object transfer protocol with more
than a single object, so multiple SSH connections are not started.

We would like to create such tests, though, to help diagnose problems
such as those described in git-lfs#5880.  Therefore we change our lfs-ssh-echo
program to use the conventional file mode of 0666 when creating its
temporary file, which will allow subsequent invocations of the problem
with ControlMaster set to "no" to also open the same file.

In addition, we alter the way we handle errors when the file can not be
created or opened, so that we output distinct error messages in the
cases where the file already exists or does not exist, respectively,
as well as in the cases where some other type of error occurs.
Previously, we assumed any error must be due to a file either existing
or not existing, and did not report the actual error message, so problems
such as those caused by missing file permissions were obscured.

In commit b44cbe4 of PR git-lfs#5136 the
"multiplex" argument was added to the FormatArgs() function in our
ssh/ssh.go source file.  Later, the "controlPath" argument was also
added, in commit 448b0c4 of PR git-lfs#5537.

Neither of these arguments is used in the function's body, though,
so we can remove them now.

In commit 448b0c4 of PR git-lfs#5537 our
lfs-ssh-echo test helper utility was updated to examine the value of
any provided ControlMaster command-line argument and simulate the action
of the OpenSSH program when it is passed a ControlMaster argument with
a value of either "yes" or "no".  (The other values OpenSSH supports
for its ControlMaster option are not accepted by our lfs-ssh-echo
utility at the moment.)

When a ControlMaster command-line argument is found, and has the value
"yes", the lfs-ssh-echo program attempts to create a temporary file at
the location given by the ControlPath command-line argument, which must
also be provided.  If the file already exists, or some other error
occurs while creating it, the program halts with a non-zero exit code.

If the ControlMaster argument's value is "no", and the ControlPath
argument is also defined, the program attempts to open the file at
the given path, and exits with a non-zero code if the file does not
already exist or some other error occurs.

This logic is designed to emulate the behaviour of OpenSSH, which
supports the use of ControlMaster and ControlPath arguments to multiplex
connections over a common control socket.  When the ControlMaster argument
is "yes", OpenSSH creates a socket, which may then be shared by other
invocations of OpenSSH by setting ControlMaster to "no" and passing
the socket's path in the ControlPath argument.

Our lfs-ssh-echo program, however, creates its temporary file with
no defined file permissions (i.e., a file mode argument of zero).
This results in a file which can not be opened by any other instances
of the lfs-ssh-echo program.  At the moment this does not cause any
problems with our test suite, because we have no tests which exercise
the SSH version of the Git LFS object transfer protocol with more
than a single object, so multiple SSH connections are not started.

We would like to create such tests, though, to help diagnose problems
such as those described in git-lfs#5880.  Therefore we change our lfs-ssh-echo
program to use the conventional file mode of 0666 when creating its
temporary file, which will allow subsequent invocations of the problem
with ControlMaster set to "no" to also open the same file.

In addition, we alter the way we handle errors when the file can not be
created or opened, so that we output distinct error messages in the
cases where the file already exists or does not exist, respectively,
as well as in the cases where some other type of error occurs.
Previously, we assumed any error must be due to a file either existing
or not existing, and did not report the actual error message, so problems
such as those caused by missing file permissions were obscured.

In commit b44cbe4 of PR git-lfs#5136 the
"multiplex" argument was added to the FormatArgs() function in our
ssh/ssh.go source file.  Later, the "controlPath" argument was also
added, in commit 448b0c4 of PR git-lfs#5537.

Neither of these arguments is used in the function's body, though,
so we can remove them now.

In commit 448b0c4 of PR git-lfs#5537 our
lfs-ssh-echo test helper utility was updated to examine the value of
any provided ControlMaster command-line argument and simulate the action
of the OpenSSH program when it is passed a ControlMaster argument with
a value of either "yes" or "no".  (The other values OpenSSH supports
for its ControlMaster option are not accepted by our lfs-ssh-echo
utility at the moment.)

When a ControlMaster command-line argument is found, and has the value
"yes", the lfs-ssh-echo program attempts to create a temporary file at
the location given by the ControlPath command-line argument, which must
also be provided.  If the file already exists, or some other error
occurs while creating it, the program halts with a non-zero exit code.

If the ControlMaster argument's value is "no", and the ControlPath
argument is also defined, the program attempts to open the file at
the given path, and exits with a non-zero code if the file does not
already exist or some other error occurs.

This logic is designed to emulate the behaviour of OpenSSH, which
supports the use of ControlMaster and ControlPath arguments to multiplex
connections over a common control socket.  When the ControlMaster argument
is "yes", OpenSSH creates a socket, which may then be shared by other
invocations of OpenSSH by setting ControlMaster to "no" and passing
the socket's path in the ControlPath argument.

Our lfs-ssh-echo program, however, creates its temporary file with
no defined file permissions (i.e., a file mode argument of zero).
This results in a file which can not be opened by any other instances
of the lfs-ssh-echo program.  At the moment this does not cause any
problems with our test suite, because we have no tests which exercise
the SSH version of the Git LFS object transfer protocol with more
than a single object, so multiple SSH connections are not started.

We would like to create such tests, though, to help diagnose problems
such as those described in git-lfs#5880.  Therefore we change our lfs-ssh-echo
program to use the conventional file mode of 0666 when creating its
temporary file, which will allow subsequent invocations of the problem
with ControlMaster set to "no" to also open the same file.

In addition, we alter the way we handle errors when the file can not be
created or opened, so that we output distinct error messages in the
cases where the file already exists or does not exist, respectively,
as well as in the cases where some other type of error occurs.
Previously, we assumed any error must be due to a file either existing
or not existing, and did not report the actual error message, so problems
such as those caused by missing file permissions were obscured.

In commit b44cbe4 of PR git-lfs#5136 the
"multiplex" argument was added to the FormatArgs() function in our
ssh/ssh.go source file.  Later, the "controlPath" argument was also
added, in commit 448b0c4 of PR git-lfs#5537.

Neither of these arguments is used in the function's body, though,
so we can remove them now.

In commit 448b0c4 of PR git-lfs#5537 our
lfs-ssh-echo test helper utility was updated to examine the value of
any provided ControlMaster command-line argument and simulate the action
of the OpenSSH program when it is passed a ControlMaster argument with
a value of either "yes" or "no".  (The other values OpenSSH supports
for its ControlMaster option are not accepted by our lfs-ssh-echo
utility at the moment.)

When a ControlMaster command-line argument is found, and has the value
"yes", the lfs-ssh-echo program attempts to create a temporary file at
the location given by the ControlPath command-line argument, which must
also be provided.  If the file already exists, or some other error
occurs while creating it, the program halts with a non-zero exit code.

If the ControlMaster argument's value is "no", and the ControlPath
argument is also defined, the program attempts to open the file at
the given path, and exits with a non-zero code if the file does not
already exist or some other error occurs.

This logic is designed to emulate the behaviour of OpenSSH, which
supports the use of ControlMaster and ControlPath arguments to multiplex
connections over a common control socket.  When the ControlMaster argument
is "yes", OpenSSH creates a socket, which may then be shared by other
invocations of OpenSSH by setting ControlMaster to "no" and passing
the socket's path in the ControlPath argument.

Our lfs-ssh-echo program, however, creates its temporary file with
no defined file permissions (i.e., a file mode argument of zero).
This results in a file which can not be opened by any other instances
of the lfs-ssh-echo program.  At the moment this does not cause any
problems with our test suite, because we have no tests which exercise
the SSH version of the Git LFS object transfer protocol with more
than a single object, so multiple SSH connections are not started.

We would like to create such tests, though, to help diagnose problems
such as those described in git-lfs#5880.  Therefore we change our lfs-ssh-echo
program to use the conventional file mode of 0666 when creating its
temporary file, which will allow subsequent invocations of the problem
with ControlMaster set to "no" to also open the same file.

In addition, we alter the way we handle errors when the file can not be
created or opened, so that we output distinct error messages in the
cases where the file already exists or does not exist, respectively,
as well as in the cases where some other type of error occurs.
Previously, we assumed any error must be due to a file either existing
or not existing, and did not report the actual error message, so problems
such as those caused by missing file permissions were obscured.

In commit 448b0c4 of PR git-lfs#5537 our
lfs-ssh-echo test helper utility was updated to examine the value of
any provided ControlMaster command-line argument and simulate the action
of the OpenSSH program when it is passed a ControlMaster argument with
a value of either "yes" or "no".  (The other values OpenSSH supports
for its this option are not accepted by our lfs-ssh-echo utility at
the moment.)

When a ControlMaster command-line argument is found, and has the value
"yes", the lfs-ssh-echo program attempts to create a temporary file at
the location given by the ControlPath command-line argument, which must
also be provided.  If the file already exists, or some other error
occurs while creating it, the program halts with a non-zero exit code.

If the ControlMaster argument's value is "no", and the ControlPath
argument is also defined, the program attempts to open the file at
the given path, and exits with a non-zero code if the file does not
already exist or some other error occurs.

This logic is designed to emulate the behaviour of OpenSSH, which
supports the use of these arguments to multiplex connections over a
common control socket.  When the ControlMaster argument is "yes",
OpenSSH creates a socket, which may then be shared by other invocations
of OpenSSH by setting the argument to "no" and passing the socket's
path in the ControlPath argument.

Our lfs-ssh-echo program, however, creates its temporary file with
no defined file permissions (i.e., a file mode argument of zero).
This results in a file which can not be opened by any other instances
of the lfs-ssh-echo program.  At the moment this does not cause any
problems with our test suite, because we have no tests which exercise
the SSH version of the Git LFS object transfer protocol with more
than a single object, so multiple SSH connections are not started.

We would like to create such tests, though, to help diagnose problems
such as those described in git-lfs#5880.  Therefore we change our lfs-ssh-echo
program to use the conventional file mode of 0666 when creating its
temporary file, which will allow subsequent invocations of the problem
with ControlMaster set to "no" to also open the same file.

In addition, we alter the way we handle errors when the file can not be
created or opened, so that we output distinct error messages in the
cases where the file already exists or does not exist, respectively,
as well as in the cases where some other type of error occurs.
Previously, we assumed any error must be due to a file either existing
or not existing, and did not report the actual error message, so problems
such as those caused by missing file permissions were obscured.

In commit b44cbe4 of PR git-lfs#5136 the
"multiplex" argument was added to the FormatArgs() function in our
ssh/ssh.go source file.  Later, the "controlPath" argument was also
added, in commit 448b0c4 of PR git-lfs#5537.

Neither of these arguments is used in the function's body, though,
so we can remove them now.

In commit 448b0c4 of PR git-lfs#5537 our
lfs-ssh-echo test helper utility was updated to examine the value of
any provided ControlMaster command-line argument and simulate the action
of the OpenSSH program when it is passed a ControlMaster argument with
a value of either "yes" or "no".  (The other values OpenSSH supports
for its this option are not accepted by our lfs-ssh-echo utility at
the moment.)

When a ControlMaster command-line argument is found, and has the value
"yes", the lfs-ssh-echo program attempts to create a temporary file at
the location given by the ControlPath command-line argument, which must
also be provided.  If the file already exists, or some other error
occurs while creating it, the program halts with a non-zero exit code.

If the ControlMaster argument's value is "no", and the ControlPath
argument is also defined, the program attempts to open the file at
the given path, and exits with a non-zero code if the file does not
already exist or some other error occurs.

This logic is designed to emulate the behaviour of OpenSSH, which
supports the use of these arguments to multiplex connections over a
common control socket.  When the ControlMaster argument is "yes",
OpenSSH creates a socket, which may then be shared by other invocations
of OpenSSH by setting the argument to "no" and passing the socket's
path in the ControlPath argument.

Our lfs-ssh-echo program, however, creates its temporary file with
no defined file permissions (i.e., a file mode argument of zero).
This results in a file which can not be opened by any other instances
of the lfs-ssh-echo program.  At the moment this does not cause any
problems with our test suite, because we have no tests which exercise
the SSH version of the Git LFS object transfer protocol with more
than a single object, so multiple SSH connections are not started.

We would like to create such tests, though, to help diagnose problems
such as those described in git-lfs#5880.  Therefore we change our lfs-ssh-echo
program to use the conventional file mode of 0666 when creating its
temporary file, which will allow subsequent invocations of the problem
with ControlMaster set to "no" to also open the same file.

In addition, we alter the way we handle errors when the file can not be
created or opened, so that we output distinct error messages in the
cases where the file already exists or does not exist, respectively,
as well as in the cases where some other type of error occurs.
Previously, we assumed any error must be due to a file either existing
or not existing, and did not report the actual error message, so problems
such as those caused by missing file permissions were obscured.

In commit 448b0c4 of PR git-lfs#5537 our
lfs-ssh-echo test helper utility was updated to examine the value of
any provided ControlMaster command-line argument and simulate the action
of the OpenSSH program when it is passed a ControlMaster argument with
a value of either "yes" or "no".  (The other values OpenSSH supports
for this option are not accepted by our lfs-ssh-echo utility at the
moment.)

When a ControlMaster command-line argument is found, and has the value
"yes", the lfs-ssh-echo program attempts to create a temporary file at
the location given by the ControlPath command-line argument, which must
also be provided.  If the file already exists, or some other error
occurs while creating it, the program halts with a non-zero exit code.

If the ControlMaster argument's value is "no", and the ControlPath
argument is also defined, the program attempts to open the file at
the given path, and exits with a non-zero code if the file does not
already exist or some other error occurs.

This logic is designed to emulate the behaviour of OpenSSH, which
supports the use of these arguments to multiplex SSH sessions over
a common connection using a control socket.  When the ControlMaster
argument is "yes", OpenSSH creates a socket, which may then be shared
with other invocations of OpenSSH by setting the ControlMaster argument
to "no" and passing the socket's path in the ControlPath argument.

Our lfs-ssh-echo program, however, creates its temporary file with
no defined file permissions (i.e., with a file mode argument of zero).
This results in a file which can not be opened by any other instances
of the lfs-ssh-echo program.  At the moment this does not cause any
problems with our test suite, because we have no tests which exercise
the SSH version of the Git LFS object transfer protocol with more
than a single object, so multiple SSH sessions with the same
ControlPath argument are never started.

We would like to create additional tests which push and fetch multiple
Git LFS objects over SSH, though, to help diagnose issues such as
those described in git-lfs#5880.  Therefore we change our lfs-ssh-echo
program to use the conventional file mode of 0666 when creating its
temporary file, which will allow subsequent invocations of the program
with the ControlMaster argument set to "no" to also open the same file.

As well as fixing this bug, we also alter the way the lfs-ssh-echo
program handles errors when the file specified by the ControlPath
argument can not be created or opened.  We now output distinct error
messages in the cases where the file already exists or does not exist,
respectively, as well as in the cases where some other type of error
occurs.  Previously, we assumed any error must be due to a file either
existing or not existing, and did not report the actual error message,
so problems such as those caused by missing file permissions were
obscured.

In commit b44cbe4 of PR git-lfs#5136 the
"multiplex" argument was added to the FormatArgs() function in our
ssh/ssh.go source file.  Later, the "controlPath" argument was also
added, in commit 448b0c4 of PR git-lfs#5537.

Neither of these arguments is used in the function's body, though,
so we can remove them now.

Since commit 448b0c4 in PR git-lfs#5537
the GetExeAndArgs() function in our "ssh" package sets the default
value of our "lfs.ssh.autoMultiplex" configuration option to "false"
when running on Windows, and "true" otherwise.

This choice was made because the SSH clients available on Windows
may not support multiplexing SSH sessions over a single connection,
as OpenSSH does with its ControlMaster and ControlPath options.

Since some of these SSH clients may fail if they are passed the
ControlMaster and ControlPath options, we require Windows users who
want to use SSH multiplexing to explicitly enable it by setting the
"lfs.ssh.autoMultiplex" option to "true".  See also the discussion in:

  git-lfs#5537 (comment)

However, our git-lfs-config(5) manual page was not updated in PR git-lfs#5537
to reflect the change in the default value of the "lfs.ssh.autoMultiplex"
option on Windows, so we update it now.

Note that users with the Git for Windows project installed will
typically have a version of OpenSSH available which supports the
ControlMaster option.  However, the OpenSSH for Windows client may
not support multiplexing, as noted in PowerShell/Win32-OpenSSH#1328.