Fix bug #529968 by renaming the kept file on content conflicts.

* bzrlib/conflicts.py:
(ContentsConflict._take_it): Helper to resolve the conflict,
factored out of the two symmetric cases. Use a tree transform to
transparently handle more cases.
(ContentsConflict.action_take_this,
ContentsConflict.action_take_other):

* bzrlib/tests/test_conflicts.py:
(load_tests): Start parametrizing tests.
(content_conflict_scenarios): First scenarios for ContenConflict.
(TestResolveContentConflicts.setUp): Actions are parameters.
(TestResolveContentConflicts.test_resolve_taking_this,
TestResolveContentConflicts.test_resolve_taking_other): Finer
checks are parameters.
(TestResolveContentConflicts._get_actions)
(TestResolveContentConflicts._get_check): Helpers.
(TestResolveContentConflicts.do_modify_file,
TestResolveContentConflicts.do_delete_file): Specific actions.
(TestResolveContentConflicts.check_file_has_more_content,
TestResolveContentConflicts.check_file_doesnt_exist): Specific
checks.