remote: make guess_remote_head() use exact HEAD lookup if it is available

Our usual method for determining the ref pointed to by HEAD
is to compare HEAD's sha1 to the sha1 of all refs, trying to
find a unique match.

However, some transports actually get to look at HEAD
directly; we should make use of that information when it is
available.  Currently, only http remotes support this
feature.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJay Soffian <jaysoffian@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

remote.c

@@ -1476,6 +1476,14 @@ struct ref *guess_remote_head(const struct ref *head,
 	if (!head)
 		return NULL;
 
+	/*
+	 * Some transports support directly peeking at
+	 * where HEAD points; if that is the case, then
+	 * we don't have to guess.
+	 */
+	if (head->symref)
+		return copy_ref(find_ref_by_name(refs, head->symref));
+
 	/* If refs/heads/master could be right, it is. */
 	if (!all) {
 		r = find_ref_by_name(refs, "refs/heads/master");

t/t5550-http-fetch.sh

@@ -42,5 +42,16 @@ test_expect_success 'fetch changes via http' '
 	test_cmp file clone/file
 '
 
+test_expect_success 'http remote detects correct HEAD' '
+	git push public master:other &&
+	(cd clone &&
+	 git remote set-head origin -d &&
+	 git remote set-head origin -a &&
+	 git symbolic-ref refs/remotes/origin/HEAD > output &&
+	 echo refs/remotes/origin/master > expect &&
+	 test_cmp expect output
+	)
+'
+
 stop_httpd
 test_done