Re: master 4803fba487 1/2: 'C-x v v' on a diff buffer commits it as a patch (bug#52349)

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 29 Aug 2022 20:05:10 +0300
> Cc: juri@jurta.org, emacs-devel@gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> > Given diffs in a
> > buffer, it should be as simple as running the Patch utility via
> > shell-command-on-region, then committing the results.  Git has a
> > special command for that, but we don't need a special command for
> > other VCSes, if they don't have the equivalent of "git apply".  (I'm
> > guessing that "git apply" simply runs Patch under the hood.)
> 
> No, 'git apply' puts the patch in a different place (index area), which 
> means our implementation doesn't need to bother with moving all existing 
> changes in the selected files somewhere else, then committing, and then 
> restoring the previously-hidden changes.

I don't follow, sorry.

The implementation in vc-git.el does this, AFAICT:

  . creates a temporary file
  . inserts the patch into the temporary file
  . calls "git apply --cached" on the patch in the temporary file
  . deletes the temporary file

This doesn't seem to be very different from invoking Patch on the
diffs.  (And using shell-command-on-region avoids the need to
explicitly create a temporary file.)

> Personally I hope we discover some popular extension to Mercurial which 
> we'll be able to use in the same way as we do Git's index area here. And 
> then say job well done and keep the less-popular and outdated backends 
> unsupported.

The index thing being the problem because Git needs to have the
changes in the index before you can commit?  Or is there any other
reason?

IOW, why cannot we simply patch the files, and then run the equivalent
of vc-checkin?