builtin-show-ref: use warning() instead of fprintf(stderr, "warning: ")

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

builtin-show-ref.c

@@ -140,7 +140,7 @@ static int exclude_existing(const char *match)
 				continue;
 		}
 		if (check_ref_format(ref)) {
-			fprintf(stderr, "warning: ref '%s' ignored\n", ref);
+			warning("ref '%s' ignored", ref);
 			continue;
 		}
 		if (!string_list_has_string(&existing_refs, ref)) {