unpack-trees: allow pruning with pathspec

unpack-trees: allow pruning with pathspec

Use the pathspec pruning of traverse_trees() from unpack_trees(). Again,
the unpack_trees() machinery is primarily meant for merging two (or more)
trees, and because a merge is a full tree operation, it didn't support any
pruning with pathspec, and this codepath probably should not be enabled
while running a merge, but the caller in diff-lib.c::diff_cache() should
be able to take advantage of it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>