mark "pointless" data pointers in callbacks

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>