snapshots: Add short ID to JSON output

We're using the short ID in all output to users, so it should also be
included in the JSON output of the `snapshots` command.

cmd/restic/cmd_snapshots.go

@@ -173,6 +173,7 @@ type Snapshot struct {
 	*restic.Snapshot
 
 	ID      *restic.ID `json:"id"`
+	ShortID string     `json:"short_id"`
 }
 
 // printSnapshotsJSON writes the JSON representation of list to stdout.
@@ -185,6 +186,7 @@ func printSnapshotsJSON(stdout io.Writer, list restic.Snapshots) error {
 		k := Snapshot{
 			Snapshot: sn,
 			ID:       sn.ID(),
+			ShortID:  sn.ID().Str(),
 		}
 		snapshots = append(snapshots, k)
 	}