Codepaths to walk tree objects have been audited for integer
overflows and hardened.

* jk/tree-walk-overflow:
  tree-walk: harden make_traverse_path() length computations
  tree-walk: add a strbuf wrapper for make_traverse_path()
  tree-walk: accept a raw length for traverse_path_len()
  tree-walk: use size_t consistently
  tree-walk: drop oid from traverse_info
  setup_traverse_info(): stop copying oid