[MERGE] Fix bug #235407, when the same revision is merged twice

[Robert Collins]

Robert Collins has voted reject.
Status is now: Vetoed
Comment:
merge is definitely doing the wrong thing and should not be any harder 
to fix: there were tests at one point that add_parent_ids would 
normalise any new parents so only the heads were listed.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C4840802E.7060900%40arbash-meinel.com%3E