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
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