* (bug 21279, bug 28820) Workaround for standard diff links to deleted revs; Special...

* (bug 21279, bug 28820) Workaround for standard diff links to deleted revs; Special:Undelete links to deleted rev when allowed.

Adds links to Special:Undelete to view the requested revisions onto the 'revision data not found' error message, if they can be found via ar_rev_id and the user has permission to deletedhistory.

Follow-up to r87804; allows the 'diff' links on log entries for Special:RevisionDelete actions on deleted revs referenced by rev-id to continue to go someplace semi-sane.
Could benefit a more transparent forwarding -> bug 28820.