changectx: search manifest delta for filenode

context: check self.__dict__ instead of using hasattr hasattr implicitly calls getattr, instantiating the field it is checking for.

Teach bdiff to support buffer objects manifest.add gives revlog.addrevision a buffer object, which may be cached and used for a second call in the same session (as mq does when pushing multiple patches). The other option would be to cast the buffer to str when caching it.