t/helper: mark unused argv/argc arguments

Many test helper programs do not bother to look at argc or argv, because
they don't take any options. In a user-facing program, it's a good idea
to check for unexpected arguments and complain. But for a test helper,
it's not worth the trouble to enforce this.

But we do want to tell the compiler we're OK with ignoring them, to
silence -Wunused-parameter (and obviously we can't get rid of them,
since we have to conform to the usual cmd__foo() interface).

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

t/helper/test-dump-split-index.c

@@ -10,7 +10,7 @@ static void show_bit(size_t pos, void *data)
 	printf(" %d", (int)pos);
 }
 
-int cmd__dump_split_index(int ac, const char **av)
+int cmd__dump_split_index(int ac UNUSED, const char **av)
 {
 	struct split_index *si;
 	int i;