Merge branch 'en/tree-walk-optim'

Merge branch 'en/tree-walk-optim'

* en/tree-walk-optim:
  diff_tree(): Skip skip_uninteresting() when all remaining paths interesting
  tree_entry_interesting(): Make return value more specific
  tree-walk: Correct bitrotted comment about tree_entry()
  Document pre-condition for tree_entry_interesting