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
$ NO_FALLBACK="env RHG_ON_UNSUPPORTED=abort"
$ cat << EOF >> $HGRCPATH
> [format]
> sparse-revlog = no
> EOF
$ hg init repo --config format.generaldelta=no --config format.usegeneraldelta=no
$ cd repo
$ (echo header; seq.py 20) > f
$ hg commit -q -Am initial
$ (echo header; seq.py 20; echo footer) > f
$ hg commit -q -Am x
$ hg update ".^"
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ (seq.py 20; echo footer) > f
$ hg commit -q -Am y
$ hg debugdeltachain f --template '{rev} {prevrev} {deltatype}\n'
0 -1 base
1 0 prev
2 1 prev
rhg works on non-generaldelta revlogs:
$ $NO_FALLBACK hg cat f -r .
1
2
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footer