I think we should change ‘--list-generations’ to not output the zeroth
generation.  Even though it could be present in the profile, users
shouldn’t be concerned about it.

Currently, ‘--delete-generations’ ignores the zeroth generation.  Other
options seem even more confusing:

1. If you don’t have any other generations and delete it, profile
   couldn’t point to anything.

2. If you delete it along with the profile, users might be confused (or
   angry).

WDYT?