Reproduce bug #531967 on various aspects.

* bzrlib/tests/test_conflicts.py:
(resolve_conflict_scenarios): Add another PathConflict test to
reproduce bug #531967 and specialize assertConflict as different
conflicts requires different checks.
(TestParametrizedResolveConflicts.setUp): Actions now also provide
more info on changed item.
(TestParametrizedResolveConflicts.assert_PathConflict,
TestParametrizedResolveConflicts.assert_ContentsConflict): New
helpers.
(TestParametrizedResolveConflicts.assertConflict): Delegate some
checks to a specialized method.
(TestParametrizedResolveConflicts): Update the action helpers to
provide more info.

* bzrlib/merge.py:
(Merge3Merger.cook_conflicts): path/conflict_path attributes seem
to be reversed for PathConflict objects.