gimp/menus/undo-menu.xml

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE ui SYSTEM "gtkuimanager.dtd">

<ui>
  <popup action="undo-popup">
    <menuitem action="edit-undo" />
    <menuitem action="edit-redo" />
    <menuitem action="edit-undo-clear" />
  </popup>
</ui>