More fun with the provided test case: continue with

hg clone -r tip . weirder
hg push weirder
hg push -f weirder
cd weirder
hg merge --debug

Notice how the different toposort of the graph causes another ancestor to be
chosen - and even one of the parents.

For reference, I posted some guesswork and suggested a solution at
http://selenic.com/pipermail/mercurial-devel/2008-October/008324.html