dirstate: move parents source of truth to dirstatemap As part of moving dirstate storage to its own class, let's move the source of truth for the dirstate parents to dirstatemap. This requires that dirstate._pl no longer be a cache, and that all sets go through dirstatemap.setparents. Differential Revision: https://phab.mercurial-scm.org/D759

dirstate: move parent reading to the dirstatemap class As part of moving dirstate storage logic to a separate class, let's move the function that reads the parents from the file. This will allow extensions to write dirstate's that store the parents in other ways. Differential Revision: https://phab.mercurial-scm.org/D758