add TODO for supporting undo tree

src/core/Buffer.cc

@@ -411,6 +411,7 @@ void Buffer::redo()
     }
     else if (m_change_operations[m_current_change_operation_index]->children.size() > 0u)
     {
+        /* TODO: support redoing a child of the undo tree other than the first */
         child_index = *m_change_operations[m_current_change_operation_index]->children.begin();
     }