data: use data.TreeWriter to serialize&write data.Tree

Always serialize trees via TreeJSONBuilder. Add a wrapper called
TreeWriter which combines serialization and saving the tree blob in the
repository. In the future, TreeJSONBuilder will have to upload tree
chunks while the tree is still serialized. This will a wrapper like
TreeWriter, so add it right now already.

The archiver.treeSaver still directly uses the TreeJSONBuilder as it
requires special handling.

cmd/restic/cmd_recover.go

@@ -134,28 +134,28 @@ func runRecover(ctx context.Context, gopts global.Options, term ui.Terminal) err
 		return ctx.Err()
 	}
 
-	tree := data.NewTree(len(roots))
-	for id := range roots {
-		var subtreeID = id
-		node := data.Node{
-			Type:       data.NodeTypeDir,
-			Name:       id.Str(),
-			Mode:       0755,
-			Subtree:    &subtreeID,
-			AccessTime: time.Now(),
-			ModTime:    time.Now(),
-			ChangeTime: time.Now(),
-		}
-		err := tree.Insert(&node)
-		if err != nil {
-			return err
-		}
-	}
-
 	var treeID restic.ID
 	err = repo.WithBlobUploader(ctx, func(ctx context.Context, uploader restic.BlobSaverWithAsync) error {
 		var err error
-		treeID, err = data.SaveTree(ctx, uploader, tree)
+		tw := data.NewTreeWriter(uploader)
+		for id := range roots {
+			var subtreeID = id
+			node := data.Node{
+				Type:       data.NodeTypeDir,
+				Name:       id.Str(),
+				Mode:       0755,
+				Subtree:    &subtreeID,
+				AccessTime: time.Now(),
+				ModTime:    time.Now(),
+				ChangeTime: time.Now(),
+			}
+			err := tw.AddNode(&node)
+			if err != nil {
+				return err
+			}
+		}
+
+		treeID, err = tw.Finalize(ctx)
 		if err != nil {
 			return errors.Fatalf("unable to save new tree to the repository: %v", err)
 		}