Bugfix: Restic forget --keep-* options now interpret "-1" as "forever"

Restic would forget snapshots that should have been kept when "-1" was
used as a value for --keep-* options. It now interprets "-1" as forever,
e.g. an option like --keep-monthly -1 will keep all monthly snapshots.

https://github.com/restic/restic/issues/2565
https://github.com/restic/restic/pull/4234