The default edit summary for an undo edit (determined by message undo) specifies the revision number that is being reverted, but that number is shown as plain text in the edit summary and page history.
It would be convenient to have this number be a wikilink to [[Special:Diff/<revid>]], so it could be clicked afterwards to navigate to the original edit (e.g. to see the author, date, or other metadata).
This should be done by editing the undo-summary key in languages/i18n/en.json.