Enhancement: Optionally skip snapshot creation if nothing changed

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

Restic now supports the `--skip-if-unchanged` option for the `backup`
command, which omits 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
