committablectx: cache _status in _poststatus
A future patch will remove the old workingctx.status which caches the status of
the working directory, therefore we now cache this status in the poststatus
hook of committablectx.
To run the tests, do:cd tests/python run-tests.pySee http://mercurial.selenic.com/wiki/WritingTests formore information on writing tests.