DiffEditor: Use nested using namespace declarations

Change-Id: Iaf695a7c95f4267b5480036dedfa0485b841ca6e
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/diffeditor/diffeditorplugin.cpp

@@ -29,8 +29,7 @@ using namespace Core;
 using namespace TextEditor;
 using namespace Utils;
 
-namespace DiffEditor {
-namespace Internal {
+namespace DiffEditor::Internal {
 
 class ReloadInput {
 public:
@@ -535,8 +534,7 @@ void DiffEditorPlugin::initialize()
     d = new DiffEditorPluginPrivate;
 }
 
-} // namespace Internal
-} // namespace DiffEditor
+} // namespace DiffEditor::Internal
 
 #ifdef WITH_TESTS