pack-objects: use finalize_object_file() to rename pack/idx/etc

In most places that write files to the object database (even packfiles
via index-pack or fast-import), we use finalize_object_file(). This
prefers link()/unlink() over rename(), because it means we will prefer
data that is already in the repository to data that we are newly
writing.

We should do the same thing in pack-objects. Even though we don't think
of it as accepting outside data (and thus not being susceptible to
collision attacks), in theory a determined attacker could present just
the right set of objects to cause an incremental repack to generate
a pack with their desired hash.

This has some test and real-world fallout, as seen in the adjustment to
t5303 below. That test script assumes that we can "fix" corruption by
repacking into a good state, including when the pack generated by that
repack operation collides with a (corrupted) pack with the same hash.
This violates our assumption from the previous adjustments to
finalize_object_file() that if we're moving a new file over an existing
one, that since their checksums match, so too must their contents.

This makes "fixing" corruption like this a more explicit operation,
since the test (and users, who may fix real-life corruption using a
similar technique) must first move the broken contents out of the way.

Note also that we now call adjust_shared_perm() twice. We already call
adjust_shared_perm() in stage_tmp_packfiles(), and now call it again in
finalize_object_file(). This is somewhat wasteful, but cleaning up the
existing calls to adjust_shared_perm() is tricky (because sometimes
we're writing to a tmpfile, and sometimes we're writing directly into
the final destination), so let's tolerate some minor waste until we can
more carefully clean up the now-redundant calls.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>