localrepo: use _manifestmatches instead of duplicating logic

basectx: add _manifestmatches method This method is a duplicate of localrepo.mfmatches and sets the stage for factoring localrepo.status into a context method that will be customizable based on inheritance and object type.

localrepo: always set the variable r in status method This is a small patch to help streamline keeping tracking of the list of files for status in a variable already called 'r' ('s' is for subrepos in this method). We now move the setting of it out of an 'if' block so that we can later refactor more code into the context objects.