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>