Implement 'cd' and 'mkdir'.
* bzrlib/tests/test_script.py:
(TestCd, TestMkdir): Test the 'cd' and 'mkdir' commands.
* bzrlib/tests/script.py:
(TestCaseWithScript._check_output): None and '' are equal as far
as we're concerned.
(TestCaseWithScript.do_cd, TestCaseWithScript.do_mkdir):
Implements cd and mkdir.