backup: add --dry-run/-n flag to show what would happen.

This can be used to check how large a backup is or validate exclusions.
It does not actually write any data to the underlying backend. This is
implemented as a simple overlay backend that accepts writes without
forwarding them, passes through reads, and generally does the minimal
necessary to pretend that progress is actually happening.

Fixes #1542

Example usage:

$ restic -vv --dry-run . | grep add
new       /changelog/unreleased/issue-1542, saved in 0.000s (350 B added)
modified  /cmd/restic/cmd_backup.go, saved in 0.000s (16.543 KiB added)
modified  /cmd/restic/global.go, saved in 0.000s (0 B added)
new       /internal/backend/dry/dry_backend_test.go, saved in 0.000s (3.866 KiB added)
new       /internal/backend/dry/dry_backend.go, saved in 0.000s (3.744 KiB added)
modified  /internal/backend/test/tests.go, saved in 0.000s (0 B added)
modified  /internal/repository/repository.go, saved in 0.000s (20.707 KiB added)
modified  /internal/ui/backup.go, saved in 0.000s (9.110 KiB added)
modified  /internal/ui/jsonstatus/status.go, saved in 0.001s (11.055 KiB added)
modified  /restic, saved in 0.131s (25.542 MiB added)
Would add to the repo: 25.892 MiB

doc/040_backup.rst

@@ -187,6 +187,23 @@ On **Windows**, a file is considered unchanged when its path, size
 and modification time match, and only ``--force`` has any effect.
 The other options are recognized but ignored.
 
+Dry Runs
+********
+
+You can perform a backup in dry run mode to see what would happen without
+modifying the repo.
+
+-  ``--dry-run``/``-n`` do not write anything, just print what would be done
+
+Combined with ``--verbose``, you can see a list of changes:
+
+.. code-block:: console
+
+    $ restic -r /srv/restic-repo backup ~/work --dry-run -vv | grep added
+    modified  /plan.txt, saved in 0.000s (9.110 KiB added)
+    modified  /archive.tar.gz, saved in 0.140s (25.542 MiB added)
+    Would be added to the repo: 25.551 MiB
+
 Excluding Files
 ***************