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-drop-caches.c

@@ -155,7 +155,7 @@ static int cmd_dropcaches(void)
 
 #endif
 
-int cmd__drop_caches(int argc, const char **argv)
+int cmd__drop_caches(int argc UNUSED, const char **argv UNUSED)
 {
 	cmd_sync();
 	return cmd_dropcaches();