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>

combine-diff.c

@@ -372,7 +372,7 @@ struct combine_diff_state {
 static void consume_hunk(void *state_,
 			 long ob, long on,
 			 long nb, long nn,
-			 const char *funcline, long funclen)
+			 const char *func UNUSED, long funclen UNUSED)
 {
 	struct combine_diff_state *state = state_;