pack-refs: prune top-level refs like "refs/foo"

After we have packed all refs, we prune any loose refs that
correspond to what we packed. We do so by first taking a
lock with lock_ref_sha1, and then deleting the loose ref
file.

However, lock_ref_sha1 will refuse to take a lock on any
refs that exist at the top-level of the "refs/" directory,
and we skip pruning the ref.  This is almost certainly not
what we want to happen here. The criteria to be pruned
should not differ from that to be packed; if a ref makes it
to prune_ref, it's because we want it both packed and
pruned (if there are refs you do not want to be packed, they
should be omitted much earlier by pack_ref_is_possible,
which we do in this case if --all is not given).

We can fix this by switching to lock_any_ref_for_update.
This behaves exactly the same with the exception of this
top-level check.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -2353,7 +2353,8 @@ static void try_remove_empty_parents(char *name)
 /* make sure nobody touched the ref, and unlink */
 static void prune_ref(struct ref_to_prune *r)
 {
-	struct ref_lock *lock = lock_ref_sha1(r->name + 5, r->sha1);
+	struct ref_lock *lock = lock_any_ref_for_update(r->name, r->sha1,
+							0, NULL);
 
 	if (lock) {
 		unlink_or_warn(git_path("%s", r->name));

t/t3210-pack-refs.sh

@@ -151,4 +151,11 @@ test_expect_success 'delete ref while another dangling packed ref' '
 	test_cmp /dev/null result
 '
 
+test_expect_success 'pack ref directly below refs/' '
+	git update-ref refs/top HEAD &&
+	git pack-refs --all --prune &&
+	grep refs/top .git/packed-refs &&
+	test_path_is_missing .git/refs/top
+'
+
 test_done