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