mark "pointless" data pointers in callbacks

Both the object_array_filter() and trie_find() functions use callback functions that let the caller specify which elements match. These callbacks take a void pointer in case the caller wants to pass in extra data. But in each case, the single user of these functions just passes NULL, and the callback ignores the extra pointer. We could just remove these unused parameters from the callback interface entirely. But it's good practice to provide such a pointer, as it guides future callers of the function in the right direction (rather than tempting them to access global data). Plus it's consistent with other generic callback interfaces. So let's instead annotate the unused parameters, in order to silence the compiler's -Wunused-parameter warning. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>