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

Testing exclude filters and other configuration options required running a
normal backup. Wrong filters could then cause files to be uploaded
unexpectedly. It was also not possible to approximately determine beforehand
how much data has to be uploaded.

We added a new --dry-run/-n option to the backup command, which performs
all the normal steps of a backup without actually writing any changes to
the repository. Passing -vv will log information about files that would
be added, allowing verification of source and exclusion backup options
without committing changes to the repository.

https://github.com/restic/restic/issues/1542
https://github.com/restic/restic/pull/2308
https://github.com/restic/restic/pull/3210
https://github.com/restic/restic/pull/3300