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/fast-export.c

@@ -409,7 +409,7 @@ static const char *anonymize_oid(const char *oid_hex)
 }
 
 static void show_filemodify(struct diff_queue_struct *q,
-			    struct diff_options *options, void *data)
+			    struct diff_options *options UNUSED, void *data)
 {
 	int i;
 	struct string_list *changed = data;