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>

diff.c

@@ -1954,7 +1954,7 @@ static int color_words_output_graph_prefix(struct diff_words_data *diff_words)
 static void fn_out_diff_words_aux(void *priv,
 				  long minus_first, long minus_len,
 				  long plus_first, long plus_len,
-				  const char *func, long funclen)
+				  const char *func UNUSED, long funclen UNUSED)
 {
 	struct diff_words_data *diff_words = priv;
 	struct diff_words_style *style = diff_words->style;
@@ -3184,8 +3184,9 @@ static int is_conflict_marker(const char *line, int marker_size, unsigned long l
 }
 
 static void checkdiff_consume_hunk(void *priv,
-				   long ob, long on, long nb, long nn,
-				   const char *func, long funclen)
+				   long ob UNUSED, long on UNUSED,
+				   long nb, long nn UNUSED,
+				   const char *func UNUSED, long funclen UNUSED)
 
 {
 	struct checkdiff_t *data = priv;