dirstate: move write into dirstatemap As part of separating the dirstate business logic from the dirstate storage logic, let's move the serialization code down into dirstatemap. Differential Revision: https://phab.mercurial-scm.org/D978

dirstate: move _read into dirstatemap As part of separating the dirstate business logic from the storage, let's move the read code into the new dirstatemap class. Differential Revision: https://phab.mercurial-scm.org/D977