merge-tree: mark unused parameter in traverse callback

merge-tree: mark unused parameter in traverse callback

Our threeway_callback() does not bother to look at its "n" parameter. It
is static in this file and used only by trivial_merge_trees(), which
always passes 3 trees (hence the name "threeway"). It also does not look
at "dirmask". This is OK, as it handles directories specifically by
looking at the mode bits.

Other traverse_info callbacks need these, so we can't get drop them from
the interface. But let's annotate these ones to avoid complaints from
-Wunused-parameter.

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