basefilectx: move _filelog from filectx

basefilectx: add an empty class that will be used as a parent of file contexts Similar to the refactoring of context, we split common logic from filectx to a parent class called basefilectx that will be inherited by filectx, workingfilectx, and memfilectx. This will allow a clear disinction of all the file contexts: - filectx: read-only access to a filerevision that is already present in the repo, - workingfilectx: a filecontext that represents files from the working directory, - memfilectx: a filecontext that represents files in-memory

workingctx: inherit from basectx instead of changectx

largefiles: remove unnecessary check of instance The refactoring of all the context objects allows us to simply pass a basectx to the __new__ constructor and have it return the same object without allocating new memory. This also removes the need to import the context module.

localrepo: remove unnecessary check of instance The refactoring of all the context objects allows us to simply pass a basectx to the __new__ constructor and have it return the same object without allocating new memory.

basectx: remove unnecessary check of instance The refactoring of all the context objects allows us to simply pass a basectx to the __new__ constructor and have it return the same object without allocating new memory.

basectx: move dirty from changectx

basectx: move dirs from changectx

basectx: move _dirs from changectx

basectx: move diff from changectx