consider only branches in guess_remote_head

The guess_remote_head function tries to figure out where a
remote's HEAD is pointing by comparing the sha1 of the
remote's HEAD with the sha1 of various refs found on the
remote. However, we were too liberal in matching refs, and
would match tags or remote tracking branches, even though
these things could not possibly be referenced by the HEAD
symbolic ref (since git will detach when checking them out).

As a result, a clone of a remote repository with a detached
HEAD might write "refs/tags/*" into our local HEAD, which is
bogus. The resulting HEAD should be detached.

The other related code path is remote.c's get_head_names()
(which is used for, among other things, "set-head -a"). This was
not affected, however, as that function feeds only refs from
refs/heads to guess_remote_head.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

remote.c

@@ -1667,7 +1667,9 @@ struct ref *guess_remote_head(const struct ref *head,
 
 	/* Look for another ref that points there */
 	for (r = refs; r; r = r->next) {
-		if (r != head && !hashcmp(r->old_sha1, head->old_sha1)) {
+		if (r != head &&
+		    !prefixcmp(r->name, "refs/heads/") &&
+		    !hashcmp(r->old_sha1, head->old_sha1)) {
 			*tail = copy_ref(r);
 			tail = &((*tail)->next);
 			if (!all)

t/t5707-clone-detached.sh

@@ -41,7 +41,7 @@ test_expect_success 'cloned HEAD matches' '
 	git --git-dir=detached-tag/.git log -1 --format=%s >actual &&
 	test_cmp expect actual
 '
-test_expect_failure 'cloned HEAD is detached' '
+test_expect_success 'cloned HEAD is detached' '
 	head_is_detached detached-tag
 '