Make all test pass by implement --strict for dpush.
* bzrlib/tests/blackbox/test_dpush.py:
Make all tests pass.
(TestDpush.test_dpush_wt_diff): Adjust the test.
* bzrlib/help_topics/en/configuration.txt:
(dpush_strict): Document the configuration variable.
* bzrlib/foreign.py:
(cmd_dpush.run): Check that the working tree has no changes when
strict is on.