shelve: teach new shelf class to check if .shelve file exists This removes the only remaining use for `shelvedfile`, so the class now goes away. Differential Revision: https://phab.mercurial-scm.org/D9713

shelve: move method for creating backup to new shelf class Differential Revision: https://phab.mercurial-scm.org/D9712

shelve: make gennames() helper generate relative backup paths When I saw `gennames()`, I thought it was meant to take a relative filename as argument. Maybe it was or maybe it wasn't, but it seems simpler to pass it a relative path anyway, so that's what this patch does. That also makes the call to backupvfs.exists() more natural (I expect a relative path to be passed there too). Differential Revision: https://phab.mercurial-scm.org/D9711

shelve: use listshelves() in cleanup function The point of this patch is to make it so all the callers of `shelvedfile.movetobackup()` look the same, so I can move it over to the new `Shelf` class next. Differential Revision: https://phab.mercurial-scm.org/D9710

shelve: inline shelvedfile.filename() since there are no callers outside class Differential Revision: https://phab.mercurial-scm.org/D9709

shelve: make listshelves() return shelf names instead of filenames All three callers now prefer the shelf name over the filename (already before my recent patches, two out of three callers preferred the shelf name). Differential Revision: https://phab.mercurial-scm.org/D9708

shelve: move method for getting stat (mtime) to new shelf class Only the mtime was needed, so I made it restricted to that in the move. The new `Shelf` class expects its argument to be a shelf name (not a arbitrary filename like `shelvedfile` would accept), so only the shelf name is now passed in. Differential Revision: https://phab.mercurial-scm.org/D9707

shelve: open patch using new shelf class instead of open() For some reason the existing code didn't use `shelvedfile().opener()` so here we migrate to `shelf().open_patch()` from the `open()` system call instead. Differential Revision: https://phab.mercurial-scm.org/D9706

shelve: move function for opening .patch file to new shelf class The `opener()` method was used specifically for the `.patch` file, and the new `Shelf` class deals with all files involved in a shelf, so I renamed the function in the process. Differential Revision: https://phab.mercurial-scm.org/D9705

shelve: move method for reading .hg to new shelf class Differential Revision: https://phab.mercurial-scm.org/D9704