commit: run git gc --auto just before the post-commit hook

commit: run git gc --auto just before the post-commit hook

Change the behavior of git-commit back to what it was back in
d4bb43ee27 ("Invoke "git gc --auto" from commit, merge, am and
rebase.", 2007-09-05) when it was git-commit.sh.

Shortly afterwards in f5bbc3225c ("Port git commit to C.", 2007-11-08)
when it was ported to C, the "git gc --auto" invocation went away.

Since that unintended regression, git gc --auto only ran for git-am,
git-merge, git-fetch, and git-receive-pack.  It was possible to
write a script that would "git commit" a lot of data locally, and gc
would never run.

One such repository that was locally committing generated zone file
changes had grown to a size of ~60GB before a daily cronjob was added
to "git gc", bringing it down to less than 1GB. This will make such
cases work without intervention.

I think fixing such pathological cases where the repository will grow
forever is a worthwhile trade-off for spending a couple of
milliseconds calling "git gc --auto" (in the common cases where it
doesn't do anything).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>