Use VcsBaseClient::diff instead of cvs's one

The second step in cleaning a mess in VCS

Change-Id: I3eb360825480c74242110b0da9f90b39fc4c767f
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>

src/plugins/cvs/cvscontrol.cpp

@@ -55,7 +55,7 @@ Core::Id CvsControl::id() const
 
 bool CvsControl::isConfigured() const
 {
-    const QString binary = m_plugin->settings().cvsBinaryPath;
+    const QString binary = m_plugin->settings().binaryPath();
     if (binary.isEmpty())
         return false;
     QFileInfo fi(binary);