dirstate: create new dirstatemap class This is part of a larger refactor to move the dirstate storage logic to a separate class, so it's easier to rewrite the dirstate storage layer without having to rewrite all the algorithms as well. Step one it to create the class, and replace dirstate._map with it. The abstraction bleeds through in a few places where the main dirstate class has to access self._map._map, but those will be cleaned up in future patches. Differential Revision: https://phab.mercurial-scm.org/D752