merge: remove direct rustmod reference
We shouldn't rely on this member being present in `dirstate.py`, this creates
unnecessary coupling.
This also can trigger certain issues in edge-cases where the policy is changed
at runtime or multiple Python environments fight, which is an added bonus.
Differential Revision: https://phab.mercurial-scm.org/D12217
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.