Make inheritance clearer in size() and add()

Reintroduce simpler kwfilelog.add(); simplify kwfilelog.read() Without add() strange things happen to changectx().

Use self._path directly instead of assigning f to context.path()

Override filelog's size(), cmp() as well; remove add() Try whether we /really/ don't need add(). size() and cmp() are needed to avoid eg: $ hg st $ touch file $ hg st M file