Log Message:
py-dulwich: updated to 0.18.6

0.18.6:
BUG FIXES
* Fix handling of empty repositories in ``porcelain.clone``.
* Raise an error when attempting to add paths that are not under the
  repository.
* Fix error message for missing trailing ].

IMPROVEMENTS
* Enforce date field parsing consistency. This also add checks on
  those date fields for potential overflow.