Add ``bzrlib.transform.orphan_policy`` and allows ``never`` to restore the previous behaviour.
* bzrlib/transform.py: (TreeTransformBase._get_potential_orphans): Simplified. (DiskTreeTransform.new_orphan): Use the orphaning_registry. (refuse_orphan): Implement the previous behaviour (never orphan). (conflict_pass): Clarify orphan creation and fallback on error.
* bzrlib/tests/test_transform.py: (TestOrphan.test_never_orphan): Add more tests to cover no orphaning and orphaning errors.