The tree:0 filter does not need to traverse the trees that it has
filtered out, so optimize list-objects and list-objects-filter to skip
traversing the trees entirely. Before this patch, we iterated over all
children of the tree, and did nothing for all of them, which was
wasteful.
Signed-off-by: Matthew DeVore <matvore@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>