shelve: introduce class representing a shelf I'm about to make phase-based shelve not write `.hg` and `.patch` files. Having a class that represents a single shelf, regardless of which files it uses will help. I'm starting small with just a `.exists()` function. I plan to eventually remove the `shelvedfile` class once all functionality has been moved to the new class. By the way, I know that things you shelve are not typically themselves shelves. I still picked `Shelf` for the class because it's short (compared to e.g. `ShelvedChange`). Differential Revision: https://phab.mercurial-scm.org/D9700