test-mergesort: mark unused parameters in trivial callback

The mode_copy() function does nothing, but since it's used as a function
pointer within "struct mode", it has to conform to the interface. Mark
it to quiet -Wunused-parameter.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-mergesort.c

@@ -122,7 +122,7 @@ static const struct dist *get_dist_by_name(const char *name)
 	return NULL;
 }
 
-static void mode_copy(int *arr, int n)
+static void mode_copy(int *arr UNUSED, int n UNUSED)
 {
 	/* nothing */
 }