describe: Use for_each_rawref

Don't waste time checking for dangling refs; they wouldn't affect the
output of 'git describe' anyway.  Although this does not gain much
performance by itself, it does in conjunction with the next commits.
Signed-off-by: NAnders Kaseorg <andersk@ksplice.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/describe.c

@@ -418,7 +418,7 @@ int cmd_describe(int argc, const char **argv, const char *prefix)
 		return cmd_name_rev(i + argc, args, prefix);
 	}
 
-	for_each_ref(get_name, NULL);
+	for_each_rawref(get_name, NULL);
 	if (!found_names && !always)
 		die("No names found, cannot describe anything.");