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

Testing exclude filters and other configuration options was error prone as
wrong filters could cause files to be uploaded unintentionally. It was also
not possible to estimate beforehand how much data would be uploaded.

The `backup` command now has a `--dry-run`/`-n` option, which performs all the
normal steps of a backup without actually writing anything to the repository.

Passing -vv will log information about files that would be added, allowing for
verification of source and exclusion options before running the real backup.

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
