Only generate the list of snapshots when
a) the argument must be a snapshot and nothing else, or
b) the argument as typed contains a literal @, or
c) a snapshot is a valid completion and there is only one dataset
matching the argument as entered.
Unfortunately, it seems the `zfs` command itself is extremely primitive
and doesn't support listing snapshots by dataset so when we need to
generate completions, we end up needing to enumerate all snapshots
(ever) across all datasets. I'd be very happy to be proven wrong, but I
think the only other way would be manually parse `zdb` output.
See #7472
