SVN: Always use internal diff command

This unbreaks svn diff views for users with custom diff
commands set up in their subversion configuration.

Task-number: QTCREATORBUG-9303
Change-Id: Ie3a726f08ad39770b63e2432755467df429067f8
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/subversion/subversionplugin.cpp

@@ -572,6 +572,7 @@ void SubversionPlugin::svnDiff(const Subversion::Internal::SubversionDiffParamet
                              QFileInfo(p.files.front()).fileName() : p.diffName;
 
     QStringList args(QLatin1String("diff"));
+    args.append(QLatin1String("--internal-diff"));
     args.append(p.arguments);
     args << p.files;