bug#73320: [PATCH] project--vc-list-files: use Git's sparse-index

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 73320@debbugs.gnu.org
> Date: Thu, 19 Sep 2024 01:27:03 +0300
> From: Dmitry Gutov <dmitry@gutov.dev>
> 
> -                      (unless (member file submodules)
> +                      (unless (or (member file submodules)
> +                                  (eq ?/ (aref file (1- (length file)))))
                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Why not use directory-name-p here?

> > Incidentally looking at the version check within `project-files`, it's
> > worthwhile to point out that `--sparse` is likely /not/ compatible with
> > ancient versions of Git. Does vc have any sort of policy on requiring
> > recent versions of these tools? If the answer is 'not really', I'll
> > additionally want to add some sort of protection against using
> > `--sparse` when the Git version won't understand it. This should be easy
> > enough to do within the implementation of `project--vc-list-files`.
> 
> IIRC it was something like "should work on the CentOS stable", and maybe 
> CentOS N-1 as well. But the release-based distro was discontinued since 
> the last time this question came up ;-(
> 
> We can call vc-git--program-version the same way it's used in 
> vc-git-state. Which version should we make the minimum?

In which version was --sparse support introduced, and what is
considered "ancient" for this purpose?  I would not like us to rely on
shining new features of Git if the price is to break people who have
older versions.  TRT in these cases is to probe the version and act
accordingly, using the new features only when they are supported.