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/ui/json/backup.go

@@ -34,6 +34,7 @@ type Backup struct {
 	term  *termstatus.Terminal
 	v     uint
 	start time.Time
+	dry   bool
 
 	totalBytes uint64
 
@@ -403,6 +404,7 @@ func (b *Backup) Finish(snapshotID restic.ID) {
 		TotalBytesProcessed: b.summary.ProcessedBytes,
 		TotalDuration:       time.Since(b.start).Seconds(),
 		SnapshotID:          snapshotID.Str(),
+		DryRun:              b.dry,
 	})
 }
 
@@ -412,6 +414,11 @@ func (b *Backup) SetMinUpdatePause(d time.Duration) {
 	b.MinUpdatePause = d
 }
 
+// SetDryRun marks the backup as a "dry run".
+func (b *Backup) SetDryRun() {
+	b.dry = true
+}
+
 type statusUpdate struct {
 	MessageType      string   `json:"message_type"` // "status"
 	SecondsElapsed   uint64   `json:"seconds_elapsed,omitempty"`
@@ -457,4 +464,5 @@ type summaryOutput struct {
 	TotalBytesProcessed uint64  `json:"total_bytes_processed"`
 	TotalDuration       float64 `json:"total_duration"` // in seconds
 	SnapshotID          string  `json:"snapshot_id"`
+	DryRun              bool    `json:"dry_run,omitempty"`
 }