revlog: introduce an explicit NodeMap class for pure code This class make the "pure" nodemap raise the same exception than the C-extension one. This is a step toward unifying nodemap and index, the class is not meant to survive on the long run. This work is part of a refactoring to unify the revlog index and the nodemap. This unification prepare the use of a persistent nodemap. There is a new `isinstance` call, it will be cleaned up in coming changesets. Differential Revision: https://phab.mercurial-scm.org/D7312