VcsBase & dependent: Fix const correctness

And some minor cleanups.

Change-Id: Id0c2df6865ba84c054f0fb97c0ac42a76a128355
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/cvs/cvsplugin.cpp

@@ -149,9 +149,8 @@ public:
 
     QStringList arguments() const override
     {
-        QStringList args = m_settings.diffOptions.value().split(' ', Qt::SkipEmptyParts);
-        args += VcsBaseEditorConfig::arguments();
-        return args;
+        return m_settings.diffOptions.value().split(' ', Qt::SkipEmptyParts)
+               + VcsBaseEditorConfig::arguments();
     }
 
 private: