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
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