``bzr add`` won't blindly add conflict related files.

* bzrlib/tests/per_workingtree/test_smart_add.py:
(TestSmartAddConflictRelatedFiles): Make sure we don't add blindly
but that we can add on demand.

* bzrlib/mutabletree.py:
(MutableTree.smart_add): Check that we don't, implicitly, add a
file generated to help resolve a conflict.

* bzrlib/conflicts.py:
(Conflict.associated_filenames): New helper.
(Conflict.cleanup): Use the helper to reduce duplication.
(PathConflict, ContentsConflict, TextConflict, HandledConflict):
Define associated_filenames instead of cleanup.