dirstate: move filefoldmap to dirstatemap As part of moving the dirstate storage logic to a separate class, lets move the filfoldmap computation to that class. This will allow extensions to replace the dirstate storage with something that persists the filefoldmap. Differential Revision: https://phab.mercurial-scm.org/D754

dirstate: move nonnormalentries to dirstatemap As part of moving dirstate storage to its own class, let's move the nonnormalentries logic onto the dirstatemap class. This will let extensions replace the nonnormalentries logic with a persisted cache. Differential Revision: https://phab.mercurial-scm.org/D753

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

style: always use `x is not None` instead of `not x is None` Differential Revision: https://phab.mercurial-scm.org/D842