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