basectx: preserve loading the cached manifest in _prestatus This is just a copy from localrepo.status and is a small step to removing that method entirely. The prestatus hook is only called for changectx's, thereby ensuring that the same behavior is guaranteed.

localrepo: use new subrev method of context.py With the machinery in place, we use context.subrev instead of testing for a workingctx directly. This allows more flexibility for later patches that will add memctx to the mix.

committablectx: add subrev method to return None This allows a future patch to use object oriented style to remove an if statement in the status method.