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