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

internal/repository/repository.go

@@ -10,6 +10,7 @@ import (
 	"sync"
 
 	"github.com/restic/chunker"
+	"github.com/restic/restic/internal/backend/dryrun"
 	"github.com/restic/restic/internal/cache"
 	"github.com/restic/restic/internal/crypto"
 	"github.com/restic/restic/internal/debug"
@@ -72,6 +73,11 @@ func (r *Repository) UseCache(c *cache.Cache) {
 	r.be = c.Wrap(r.be)
 }
 
+// SetDryRun sets the repo backend into dry-run mode.
+func (r *Repository) SetDryRun() {
+	r.be = dryrun.New(r.be)
+}
+
 // PrefixLength returns the number of bytes required so that all prefixes of
 // all IDs of type t are unique.
 func (r *Repository) PrefixLength(ctx context.Context, t restic.FileType) (int, error) {