expose versions field to support pruning

mutable_tree.go

@@ -270,7 +270,7 @@ func (tree *MutableTree) LoadVersionForOverwriting(targetVersion int64) (int64,
 	if err != nil {
 		return latestVersion, err
 	}
-	tree.deleteVersionsFrom(targetVersion+1)
+	tree.deleteVersionsFrom(targetVersion + 1)
 	return targetVersion, nil
 }
 
@@ -501,3 +501,7 @@ func (tree *MutableTree) addOrphans(orphans []*Node) {
 		tree.orphans[string(node.hash)] = node.version
 	}
 }
+
+func (tree *MutableTree) GetVersions() map[int64]bool {
+	return tree.versions
+}