shelve: replace repo instance in Shelf class by vfs instance I'd like to be able to teach the `Shelf` class to read shelves from `.hg/shelve-backup/` in addition to `.hg/shelved/`. That means that we'll have to pass in a vfs instead of the repo, so this patch does that preparation. Differential Revision: https://phab.mercurial-scm.org/D9738