diff: mark unused parameters in callbacks

The diff code provides a format_callback interface, but not every
callback needs each parameter (e.g., the "opt" and "data" parameters are
frequently left unused). Likewise for the output_prefix callback, the
low-level change/add_remove interfaces, the callbacks used by
xdi_diff(), etc.

Mark unused arguments in the callback implementations to quiet
-Wunused-parameter.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -1043,7 +1043,7 @@ static void prepare_submodule_summary(struct summary_cb *info,
 }
 
 static void submodule_summary_callback(struct diff_queue_struct *q,
-				       struct diff_options *options,
+				       struct diff_options *options UNUSED,
 				       void *data)
 {
 	int i;