Preserve scene unique names when saving branch as scene.

(cherry picked from commit 7814dedc91a9df70a6c873d0ad30f444c401b0ec)

editor/scene_tree_dock.cpp

@@ -275,6 +275,8 @@ void SceneTreeDock::_replace_with_branch_scene(const String &p_file, Node *base)
 		return;
 	}
 
+	instantiated_scene->set_unique_name_in_owner(base->is_unique_name_in_owner());
+
 	EditorUndoRedoManager *undo_redo = EditorUndoRedoManager::get_singleton();
 	undo_redo->create_action(TTR("Replace with Branch Scene"));