The "rev-list --filter" feature learned to exclude all trees via
"tree:0" filter.

* md/filter-trees:
  list-objects: support for skipping tree traversal
  filter-trees: code clean-up of tests
  list-objects-filter: implement filter tree:0
  list-objects-filter-options: do not over-strbuf_init
  list-objects-filter: use BUG rather than die
  revision: mark non-user-given objects instead
  rev-list: handle missing tree objects properly
  list-objects: always parse trees gently
  list-objects: refactor to process_tree_contents
  list-objects: store common func args in struct