pack-objects: rename .idx files into place after .bitmap files

pack-objects: rename .idx files into place after .bitmap files

In preceding commits the race of renaming .idx files in place before
.rev files and other auxiliary files was fixed in pack-write.c's
finish_tmp_packfile(), builtin/repack.c's "struct exts", and
builtin/index-pack.c's final(). As noted in the change to pack-write.c
we left in place the issue of writing *.bitmap files after the *.idx,
let's fix that issue.

See 7cc8f971085 (pack-objects: implement bitmap writing, 2013-12-21)
for commentary at the time when *.bitmap was implemented about how
those files are written out, nothing in that commit contradicts what's
being done here.

Note that this commit and preceding ones only close any race condition
with *.idx files being written before their auxiliary files if we're
optimistic about our lack of fsync()-ing in this are not tripping us
over. See the thread at [1] for a rabbit hole of various discussions
about filesystem races in the face of doing and not doing fsync() (and
if doing fsync(), not doing it properly).

We may want to fsync the containing directory once after renaming the
*.idx file into place, but that is outside the scope of this series.

1. https://lore.kernel.org/git/8735qgkvv1.fsf@evledraar.gmail.com/

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