Enhancement: Add --dry-run/-n option to backup command.

We added a new --dry-run/-n option to backup, which performs all the normal
steps of a backup without actually writing data. Passing -vv will log
information about files that would be added, allowing fast verification of
backup options without any unnecessary write activity.

https://github.com/restic/restic/issues/1542
https://github.com/restic/restic/pull/2308