packfile: delete .idx files before .pack files

packfile: delete .idx files before .pack files

When installing a packfile, we place the .pack file before the .idx
file. The intention is that Git scans for .idx files in the pack
directory and then loads the .pack files from that list.

However, when we delete packfiles, we do not do this in the reverse
order as we should. The unlink_pack_path() method deletes the .pack
followed by the .idx.

This creates a window where the process could be interrupted between
the .pack deletion and the .idx deletion, leaving the repository in a
state that looks strange, but isn't actually too problematic if we
assume the pack was safe to delete. The .idx without a .pack will cause
some overhead, but will not interrupt other Git processes.

This ordering was introduced into the 'git repack' builtin by
a1bbc6c0176 (repack: rewrite the shell script in C, 2013-09-15), though
we must be careful to track history through the code move in 8434e85d5f9
(repack: refactor pack deletion for future use, 2019-06-10) to see that.

This became more important after 73320e49add (builtin/repack.c: only
collect fully-formed packs, 2023-06-07) changed how 'git repack' scanned
for packfiles for use in the cruft pack process. It previously looked
for .pack files, but that was problematic due to the order that packs
are installed: repacks between the creation of a .pack and the creation
of its .idx would result in hard failures.

There is an independent proposal about what to do in the case of a .idx
without a .pack during this 'git repack' scenario, but this change is
focused on deleting .pack files more safely.

Modify the order to delete the .idx before the .pack. The rest of the
modifiers on the .pack should still come after the .pack so we know all
of the presumed properties of the packfile as long as it exists in the
filesystem, in case we wish to reinstate it by re-indexing the .pack
file.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>