Log Message:
py-dulwich: updated to 0.20.22

0.20.22
* Prevent removal of refs directory when the last ref is
  deleted.
* Fix filename: MERGE_HEADS => MERGE_HEAD.
* For ignored directories, porcelain.add and porcelain.status now only return
  the path to directory itself in the list of ignored paths. Previously, paths
  for all files within the directory would also be included in the list.
* Provide depth argument to ``determine_wants``.
* Various tag signature handling improvements.
* Add separate Tag.verify().
* Add support for version 3 index files.
* Fix autocrlf=input handling.
* Attempt to find C Git global config on Windows.

API CHANGES
* The APIs for writing and reading individual index entries have changed
  to handle lists of (name, entry) tuples rather than tuples.