Fix git branch -m for symrefs.

This had two problems with symrefs. First, it copied the actual sha1
instead of the "pointer", second it failed to remove the old ref after a
successful rename.

Given that till now delete_ref() always dereferenced symrefs, a new
parameters has been introduced to delete_ref() to allow deleting refs
without a dereference.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-branch.c

@@ -160,7 +160,7 @@ static int delete_branches(int argc, const char **argv, int force, int kinds)
 			continue;
 		}
 
-		if (delete_ref(name, sha1)) {
+		if (delete_ref(name, sha1, 0)) {
 			error("Error deleting %sbranch '%s'", remote,
 			       argv[i]);
 			ret = 1;