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/merge.c

@@ -776,7 +776,7 @@ static int try_merge_strategy(const char *strategy, struct commit_list *common,
 }
 
 static void count_diff_files(struct diff_queue_struct *q,
-			     struct diff_options *opt, void *data)
+			     struct diff_options *opt UNUSED, void *data)
 {
 	int *count = data;