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

Ancestors of 5
4 2 0 
Ancestors of 6 and 5
4 3 2 1 0 
Ancestors of 5 and 4
4 2 0 
Ancestors of 7, stop at 6
6 
Ancestors of 7, including revs
7 6 5 4 3 2 1 0 
Ancestors of 7, 5 and 3, including revs
7 6 5 4 3 2 1 0 

Descendants of 5
7 8 
Descendants of 5 and 3
6 7 8 
Descendants of 5 and 4
5 7 8