revlog: return the nodemap as the nodecache
Ultimately, the nodecache is the nodemap. To stop having this disctinction, we
start using the nodemap where the nodecache was used.
This work is part of a refactoring to unify the revlog index and the nodemap.
This unification prepare the use of a persistent nodemap.
Differential Revision: https://phab.mercurial-scm.org/D7316