workingctx: have status method call super instead of customized code. The old code is unneeded now that basectx has the ability to calculate the status between two context objects. Now, we use the objected oriented pattern called 'super'.

basectx: copy localrepo.status method Now that all the pieces are in place, we copy the status method from localrepo. In the next few patches, we will remove the old implementation.

committablectx: cache _status in _poststatus A future patch will remove the old workingctx.status which caches the status of the working directory, therefore we now cache this status in the poststatus hook of committablectx.

committablectx: simplify caching the status Previously, workingctx had custom variables for the unknown, ignored, and clean list of files of status. These then got moved to committablectx and, after the refactoring of localrepo.status, are no longer needed. We, therefore, simplify the whole mess. As a bonus, we are able to remove the need for having 'assert'.