Git: Simplify editor code a bit

* Introduce a const submitEditorWidget() and use it
* Directly use layout from ui

Change-Id: Ib566bab464d4f1c08a3f2af0edb5e1fa989c3160
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/git/gitsubmiteditor.h

@@ -71,6 +71,7 @@ private slots:
 
 private:
     inline GitSubmitEditorWidget *submitEditorWidget();
+    inline const GitSubmitEditorWidget *submitEditorWidget() const;
 
     VcsBase::SubmitFileModel *m_model;
     QString m_commitEncoding;