This fixes github.com/zsh-users/zsh-completions/issues/809 by
cherry-picking github.com/tarjoilija/pull/132

This commit fixes an issue where the following error message would show
up whenever zsh starts up [1]:

    _arguments:comparguments:325: can only be called from completion
    function

This commit fixes the issue by replacing the zgen repo used by our
dotfiles with a fork that contains a community-contributed fix [2].

This is done since the official zgen repo appears to no longer be
actively maintained [3], meaning that the community-contributed PR is
unlikely to be accepted any time soon.

In the long-term, we should consider migrating to actively maintained
forks of zgen like zgenom [4] or a different zsh plugin manager.
However, given that zsh plugin managers have a tendency to go inactive,
it might be best to consider finding a way to remove our reliance on
plugin managers entirely and just clone the zsh plugins directly.

[1] zsh-users/zsh-completions#809
[2] tarjoilija/zgen#132
[3] tarjoilija/zgen#123
[4] tarjoilija/zgen#123 (comment)

This reverts commit d942f39.

This commit was cherry-picked from github.com/tarjoilija/pull/132
as a way to fix github.com/zsh-users/zsh-completions/issues/809.

However, this commit seems to have problems [1], and a more correct
solution exists [2].

[1] tarjoilija#132 (comment)
[2] jandamm/zgenom#61

Closes tarjoilija#132