If item A once linked to dewiki:Foo but now links to dewiki:Bar, and item B now links to dewiki:Foo, item A can not be restored to the old revision that points to dewiki:Foo.
This type of global-consistency conflict should be checked when preparing the undo/restore operation, just like "local" conflicts that arise from later edits of the item. The respective change to the sitelinks should be omitted, and this should be reported to the user.
Implementation note: care should be taken that this works on the restore as well as the undo codepath - the restore codepath so far bypasses checks for conflicting edits.
Reminder: fix broken error message that is shown when a sitelink conflict causes save to fail.