TW-1704: Task: Use Task::identifier to reference the task

To achieve consistent output, use a helper method to generate a proper
handle - ID for tasks with non-zero ID, UUID otherwise.

src/commands/CmdPrepend.cpp

@@ -79,7 +79,7 @@ int CmdPrepend::execute (std::string&)
 
     // Prepend to the specified task.
     std::string question = format (STRING_CMD_PREPEND_CONFIRM,
-                                   task.id,
+                                   task.identifier (true),
                                    task.get ("description"));
 
     task.modify (Task::modPrepend, true);