refs_ref_iterator_begin(): don't check prefixes redundantly

The backend already correctly restricts its output to references whose
names start with the prefix. By passing the prefix again to
`prefix_ref_iterator`, we were forcing that iterator to do redundant
prefix comparisons. So set it to the empty string.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -1247,7 +1247,13 @@ struct ref_iterator *refs_ref_iterator_begin(
 	struct ref_iterator *iter;
 
 	iter = refs->be->iterator_begin(refs, prefix, flags);
-	iter = prefix_ref_iterator_begin(iter, prefix, trim);
+
+	/*
+	 * `iterator_begin()` already takes care of prefix, but we
+	 * might need to do some trimming:
+	 */
+	if (trim)
+		iter = prefix_ref_iterator_begin(iter, "", trim);
 
 	return iter;
 }