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-tree.c

@@ -98,7 +98,7 @@ static void *origin(struct merge_list *entry, unsigned long *size)
 	return NULL;
 }
 
-static int show_outf(void *priv_, mmbuffer_t *mb, int nbuf)
+static int show_outf(void *priv UNUSED, mmbuffer_t *mb, int nbuf)
 {
 	int i;
 	for (i = 0; i < nbuf; i++)