Merge branch 'en/dir-api-cleanup'

Code clean-up to clarify directory traversal API.

* en/dir-api-cleanup:
  unpack-trees: add usage notices around df_conflict_entry
  unpack-trees: special case read-tree debugging as internal usage
  unpack-trees: rewrap a few overlong lines from previous patch
  unpack-trees: mark fields only used internally as internal
  unpack_trees: start splitting internal fields from public API
  sparse-checkout: avoid using internal API of unpack-trees, take 2
  sparse-checkout: avoid using internal API of unpack-trees
  unpack-trees: clean up some flow control
  dir: mark output only fields of dir_struct as such
  dir: add a usage note to exclude_per_dir
  dir: separate public from internal portion of dir_struct
  unpack-trees: heed requests to overwrite ignored files
  t2021: fix platform-specific leftover cruft