Start testing and implementing --strict for dpush.
* bzrlib/tests/blackbox/test_dpush.py:
(load_tests, TestDpushStrictMixin, TestDpushStrictWithoutChanges,
TestDpushStrictWithChanges): Copied from test_push with minimal
changes to have running tests with some even passing.
* bzrlib/foreign.py:
(cmd_dpush): Add the --strict option.