context: add overlayworkingcontext and overlayworkingfilectx These two classes will be used extensively in the first in-memory merge milestone. Differential Revision: https://phab.mercurial-scm.org/D616

changegroup: rename getsubsetraw to makestream Now that nothing uses getsubsetraw except makestream, let's move the functionality into the makestream. This removes the last remaining excess changegroup creation function, getsubsetraw. Differential Revision: https://phab.mercurial-scm.org/D671