Change: Include full snapshot ID in JSON output of `backup`

We have changed the JSON output of the backup command to include the full
snapshot ID instead of just a shortened version, as the latter can be ambiguous
in some rare cases. To derive the short ID, please truncate the full ID down to
eight characters.

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