dep-14-convert: Create script to automate converting git layout to DEP-14

A script that analyzes the git layout and automatically converts it into DEP14 format with debian/latest and upstream/latest, as well as creates a minimal gbp.conf if missing, and on top of that also runs salsa commands to rename the remote branches.

If invoked without any parameters, it would merely suggest what commands to run. I plan to implement --apply later to execute the changes, but this is safer without it now in first version.

Example output:

/tmp/tmp.mKjVFfQYsa/box64 master ± ~/debian/devscripts/scripts/dep14-compliance.sh
The git repository has the followng local branches:  master upstream

In DEP14, these branches must exist in the Debian packaging repository:

* debian/latest   Used to create the *.debian.tar.xz that contains the Debian
                  packaging code from the debian/ directory, and which is
                  uploaded to Debian unstable (or occasionally to experimental).
* upstream/latest Used to create the *.orig.tar.gz that contains the unmodified
                  source code of the specific upstream release.
* pristine-tar    Optional: Contains xdelta data for making the release tarball
                  bit-for-bit identical with the original one, so that the
                  upstream *.orig.tar.gz.asc signature can be validated.

Other branches may also exist, but are not required.

-> Branch debian/latest: missing, presumably 'master' should be renamed
-> Branch upstream/latest: missing, presumably 'upstream' should be renamed
-> debian/gbp.conf configured: file missing

Run the following commands to make the repository follow DEP14:
    git branch -m master debian/latest
    salsa rename_branch debian/box64 --source-branch=master --dest-branch=debian/latest
    salsa update_projects debian/box64 --rename-head --source-branch=master --dest-branch=debian/latest
    git branch -m upstream upstream/latest
    salsa rename_branch debian/box64 --source-branch=upstream --dest-branch=upstream/latest
    git checkout debian/latest
    echo "[DEFAULT]" > debian/gbp.conf
    echo "debian-branch = debian/latest" >> debian/gbp.conf
    echo "upstream-branch = upstream/latest" >> debian/gbp.conf
    git add debian/gbp.conf
    git commit -a -m "Update git repository layout to follow DEP14"

For accurate results, ensure your local git checkout is in sync with Salsa project debian/box64.