revlog: access the nodemap through the index
The revlog.rev method now access the nodemap through self.index.nodemap,
instead of using the _nodecache attribute that we are trying to remove.
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/D7317