archiver: Restructure SaveTree to work like SaveDir

SaveTree did not use the TreeSaver but rather managed the tree
collection and upload itself. This prevents using the parallelism
offered by the TreeSaver and duplicates all related code. Using the
TreeSaver can provide some speed-ups as all steps within the backup tree
now rely on FutureNodes. This can be especially relevant for backups
with large amounts of explicitly specified files.

The main difference between SaveTree and SaveDir is, that only the
former can save tree blobs in which nodes have a different name than the
actual file on disk. This is the result of resolving name conflicts
between multiple files with the same name. The filename that must be
used within the snapshot is now passed directly to
restic.NodeFromFileInfo. This ensures that a FutureNode already contains
the correct filename.

internal/archiver/tree_saver.go

@@ -10,7 +10,7 @@ import (
 
 // TreeSaver concurrently saves incoming trees to the repo.
 type TreeSaver struct {
-	saveTree func(context.Context, *restic.TreeJSONBuilder) (restic.ID, ItemStats, error)
+	saveBlob func(ctx context.Context, t restic.BlobType, buf *Buffer) FutureBlob
 	errFn    ErrorFunc
 
 	ch chan<- saveTreeJob
@@ -18,12 +18,12 @@ type TreeSaver struct {
 
 // NewTreeSaver returns a new tree saver. A worker pool with treeWorkers is
 // started, it is stopped when ctx is cancelled.
-func NewTreeSaver(ctx context.Context, wg *errgroup.Group, treeWorkers uint, saveTree func(context.Context, *restic.TreeJSONBuilder) (restic.ID, ItemStats, error), errFn ErrorFunc) *TreeSaver {
+func NewTreeSaver(ctx context.Context, wg *errgroup.Group, treeWorkers uint, saveBlob func(ctx context.Context, t restic.BlobType, buf *Buffer) FutureBlob, errFn ErrorFunc) *TreeSaver {
 	ch := make(chan saveTreeJob)
 
 	s := &TreeSaver{
 		ch:       ch,
-		saveTree: saveTree,
+		saveBlob: saveBlob,
 		errFn:    errFn,
 	}
 
@@ -110,13 +110,26 @@ func (s *TreeSaver) save(ctx context.Context, job *saveTreeJob) (*restic.Node, I
 		}
 	}
 
-	id, treeStats, err := s.saveTree(ctx, builder)
-	stats.Add(treeStats)
+	buf, err := builder.Finalize()
 	if err != nil {
 		return nil, stats, err
 	}
 
-	node.Subtree = &id
+	b := &Buffer{Data: buf}
+	res := s.saveBlob(ctx, restic.TreeBlob, b)
+
+	sbr := res.Take(ctx)
+	if !sbr.known {
+		stats.TreeBlobs++
+		stats.TreeSize += uint64(sbr.length)
+		stats.TreeSizeInRepo += uint64(sbr.sizeInRepo)
+	}
+	// The context was canceled in the meantime, id might be invalid
+	if ctx.Err() != nil {
+		return nil, stats, ctx.Err()
+	}
+
+	node.Subtree = &sbr.id
 	return node, stats, nil
 }