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>

Author: pks-t

builtin/gc.c

@@ -1587,12 +1587,12 @@ static const struct maintenance_task tasks[] = {
 	},
 	[TASK_PACK_REFS] = {
 		.name = "pack-refs",
-		.background = maintenance_task_pack_refs,
+		.foreground = maintenance_task_pack_refs,
 		.auto_condition = pack_refs_condition,
 	},
 	[TASK_REFLOG_EXPIRE] = {
 		.name = "reflog-expire",
-		.background = maintenance_task_reflog_expire,
+		.foreground = maintenance_task_reflog_expire,
 		.auto_condition = reflog_expire_condition,
 	},
 	[TASK_WORKTREE_PRUNE] = {