repository: extract LoadTree/SaveTree

The repository has no real idea what a Tree is. So these methods never
belonged there.

internal/walker/walker_test.go

@@ -67,13 +67,22 @@ func buildTreeMap(tree TestTree, m TreeMap) restic.ID {
 // TreeMap returns the trees from the map on LoadTree.
 type TreeMap map[restic.ID]*restic.Tree
 
-func (t TreeMap) LoadTree(ctx context.Context, id restic.ID) (*restic.Tree, error) {
+func (t TreeMap) LoadBlob(ctx context.Context, tpe restic.BlobType, id restic.ID, buf []byte) ([]byte, error) {
+	if tpe != restic.TreeBlob {
+		return nil, errors.New("can only load trees")
+	}
 	tree, ok := t[id]
 	if !ok {
 		return nil, errors.New("tree not found")
 	}
 
-	return tree, nil
+	tbuf, err := json.Marshal(tree)
+	if err != nil {
+		panic(err)
+	}
+	tbuf = append(tbuf, '\n')
+
+	return tbuf, nil
 }
 
 func (t TreeMap) Connections() uint {