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