Git: Fix crash when returning to commit editor from diff

The crash happens on Linux/Xming. 2 subsequents calls to
slotRefreshCommitData occur, m_commitDataFetcher is released when the
first one is handled, and the second call dereferences NULL.

Change-Id: Ibc06fe5dd89b790ac84544f6842cbf345d118703
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/vcsbase/submiteditorwidget.cpp

@@ -583,6 +583,11 @@ void SubmitEditorWidget::setUpdateInProgress(bool value)
     updateSubmitAction();
 }
 
+bool SubmitEditorWidget::updateInProgress() const
+{
+    return d->m_updateInProgress;
+}
+
 QString SubmitEditorWidget::commitName() const
 {
     return tr("&Commit");