Fix bug #206577 by adding a --strict option to send.
* bzrlib/tests/blackbox/test_send.py: (load_tests): Test send --strict for uncommitted changes and pending merges. (TestSendBase): Factor out some common helpers. (TestSendStrict): Helpers for --strict option tests. (TestSendStrictWithoutChanges, TestSendStrictWithChanges): Test the --strict option.
* bzrlib/send.py: (send): Handle the strict option.