Git: Use Qt Creator as an editor for Git

Qt Creator runs Git in a "dumb" terminal which does not
allow opening an editor. Use Qt Creator itself as an editor
when needed.

Change-Id: Ide073299964803d02479dade24ddee0283ebd83b
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/git/gitclient.h

@@ -341,6 +341,7 @@ private:
     const QString m_msgWait;
     QSignalMapper *m_repositoryChangedSignalMapper;
     GitSettings *m_settings;
+    QString m_gitQtcEditor;
 };
 
 } // namespace Internal