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