shelve: also create class representing whole directory of shelves It's a little annoying to have to create and pass in a vfs into `listshelves()`. This patch attempts to start addressing that by creating a class that represents a directory containing shelves (the directory can be either `.hg/shelved/` or `.hg/shelve-backup/`). Differential Revision: https://phab.mercurial-scm.org/D9743

shelve: add a method for deleting shelf to new shelf class This is not necessary for my future changes, but it's more consistent to encapsulate the knowledge of the various files in the `Shelf` class. Differential Revision: https://phab.mercurial-scm.org/D9742

shelve: inline ".patch" constant now that it's only used in the Shelf class Differential Revision: https://phab.mercurial-scm.org/D9741

shelve: use listshelves() in cleanupoldbackups() With this patch, there are no more assumptions outside the `Shelf` class about which files (`.patch`, `.hg`, `.shelve`) make up a shelf. As such, this finishes the preparations for making phase-based shelve (perhaps optionally) not write the `.patch` and `.hg` files. Differential Revision: https://phab.mercurial-scm.org/D9740

shelve: make listshelves() list shelves in a given vfs Differential Revision: https://phab.mercurial-scm.org/D9739

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

shelve: use listdir() instead of readdir() when we don't need stat information Differential Revision: https://phab.mercurial-scm.org/D9737