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>

submodule.c

@@ -832,7 +832,7 @@ static void changed_submodule_data_clear(struct changed_submodule_data *cs_data)
 }
 
 static void collect_changed_submodules_cb(struct diff_queue_struct *q,
-					  struct diff_options *options,
+					  struct diff_options *options UNUSED,
 					  void *data)
 {
 	struct collect_changed_submodules_cb_data *me = data;