Change: Include full snapshot ID in JSON output of the bacukp command

We have changed the JSON output of the backup command to include the full snapshot ID instead of just a shortened version.
The latter can be ambiguous in rare cases.

To derive the short ID, truncate the full ID down to 8 characters.

https://github.com/restic/restic/issues/2724
https://github.com/restic/restic/pull/3993