Resolve the PathConflict when the file-id has been lost due to bug #531967.
* bzrlib/conflicts.py:
(PathConflict._resolve): Fix comment.
(PathConflict._get_or_infer_file_id): Get back the file-id by
searching in the ancestry.
(PathConflict.action_take_this, PathConflict.action_take_other):
When file-id is not None, we need to find it back before being
able to resolve the conflict.