builtin/maintenance: fix locking race with refs and reflogs tasks

builtin/maintenance: fix locking race with refs and reflogs tasks

As explained in the preceding commit, git-gc(1) knows to detach only
after it has already packed references and expired reflogs. This is done
to avoid racing around their respective lockfiles.

Adapt git-maintenance(1) accordingly and run the "pack-refs" and
"reflog-expire" tasks in the foreground. Note that the "gc" task has the
same issue, but the fix is a bit more involved there and will thus be
done in a subsequent commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>