tests: fix mistaken copy&paste from commit
4489e9a22763
Differential Revision: https://phab.mercurial-scm.org/D8802
manifest: kill one more instance of the old merge hash hack
Differential Revision: https://phab.mercurial-scm.org/D8766
dirstate: isolate node len dependency for the pure version
When switching to a 256bit hash function, this still needs adjustment,
but concentrates the change in one place.
Differential Revision: https://phab.mercurial-scm.org/D8815