revlog: deprecate the _nodecache attribute (API) This attribute have been the same object for a long time. Code should directly access the nodemap in case of need. (Or the final API when this will be done). 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/D7318