``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.