Enhancement: `backup` can omit snapshot creation if there was no change

The `backup` command always created a snapshot even if nothing changed
compared to the parent snapshot.

Restic now supports the `--skip-if-unchanged` option for the `backup`
command to omit creating a snapshot if the new snapshot's content would
be identical to that of the parent snapshot.

https://github.com/restic/restic/issues/662
https://github.com/restic/restic/pull/4816