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