Subsequent submit callbacks are handled incorrectly after batch is added

The problem

In \Drupal\Core\Form\FormSubmitter::executeSubmitHandlers(), there is the following code for calling all submit callbacks and handling batches appropriately (by queuing all further submit callbacks until after the batch has been executed):

    foreach ($handlers as $callback) {
      // Check if a previous _submit handler has set a batch, but make sure we
      // do not react to a batch that is already being processed (for instance
      // if a batch operation performs a
      //  \Drupal\Core\Form\FormBuilderInterface::submitForm()).
      if (($batch = &$this->batchGet()) && !isset($batch['id'])) {
        // Some previous submit handler has set a batch. To ensure correct
        // execution order, store the call in a special 'control' batch set.
        // See _batch_next_set().
        $batch['sets'][] = array('form_submit' => $callback);
        $batch['has_form_submits'] = TRUE;
      }
      else {
        call_user_func_array($form_state->prepareCallback($callback), array(&$form, &$form_state));
      }
    }

The problem: the $callback added to the batch sets if a batch was set doesn't have the necessary $form_state->prepareCallback() call. Thus, it might just be something like '::save' – which _batch_next_set() won't detect as "callable":

    if (isset($current_set['form_submit']) && ($callback = $current_set['form_submit']) && is_callable($callback)) {
      // We use our stored copies of $form and $form_state to account for
      // possible alterations by previous form submit handlers.
      $complete_form = &$batch['form_state']->getCompleteForm();
      call_user_func_array($callback, array(&$complete_form, &$batch['form_state']));
    }

The effect is, that if this notation is used, all subsequent submit callbacks will just be ignored if one sets a batch.

Suggested solution

Add the necessary $form_state->prepareCallback() call either to \Drupal\Core\Form\FormSubmitter::executeSubmitHandlers() (probably the better idea) or to _batch_next_set() (should work just as well, but would appear a bit stranger, in my opinion).