commit-graph: mark unused data parameters in generation callbacks
The compute_generation_info code uses function pointers to abstract the
get/set generation operations. Some callers don't need the extra void
data pointer, which should be annotated to appease -Wunused-parameter.
Note that we can drop the assignment of the "data" parameter in
compute_generation_numbers(), as we've just shown that neither of the
callbacks it uses will access it. This matches the caller in
ensure_generations_valid(), which already does not bother to set "data".
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>