config: mark unused callback parameters

The callback passed to git_config() must conform to a particular
interface. But most callbacks don't actually look at the extra "void
*data" parameter. Let's mark the unused parameters to make
-Wunused-parameter happy.

Note there's one unusual case here in get_remote_default() where we
actually ignore the "value" parameter. That's because it's only checking
whether the option is found at all, and not parsing its value.

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

t/helper/test-userdiff.c

@@ -12,7 +12,7 @@ static int driver_cb(struct userdiff_driver *driver,
 	return 0;
 }
 
-static int cmd__userdiff_config(const char *var, const char *value, void *cb)
+static int cmd__userdiff_config(const char *var, const char *value, void *UNUSED(cb))
 {
 	if (userdiff_config(var, value) < 0)
 		return -1;