traverse_trees(): allow pruning with pathspec

traverse_trees(): allow pruning with pathspec

The traverse_trees() machinery is primarily meant for merging two (or
more) trees, and because a merge is a full tree operation, it doesn't
support any pruning with pathspec.

Since d1f2d7e (Make run_diff_index() use unpack_trees(), not read_tree(),
2008-01-19), however, we use unpack_trees() to traverse_trees() callchain
to perform "diff-index", which could waste a lot of work traversing trees
outside the user-supplied pathspec, only to discard at the blob comparison
level in diff-lib.c::oneway_diff() which is way too late.

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