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
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