Code review comment for lp:~lifeless/bzr/apply-inventory-delta

Ian report a test failure;
ERROR: per_repository.test_fetch.TestFetchSameRepository.test_fetch_to_rich_root_set_parent_1_parent_different_id_gone(RepositoryFormatCHK2)
#
    An inconsistent delta was supplied: [(('', ''), None, None), (None, ('', ''), 'my-root')]
#
reason: New items are already in the map [(('', ''), 'TREE_ROOT')].

This is a bogus delta new checks have uncovered; we need to fix the cause of this.