Junio-

Thank you for the clarification.  There is a lot of misleading
information about this on the Internet.

I was mistaken about when a merge driver is called.  Now I see that
the merge driver is called anytime that no two of
{base,parent1,parent2} are the same.  This means that a merge driver
*can* prevent a clean-but-incorrect merge.  I was right that a
mergetool cannot be used to correct a clean-but-incorrect merge.

-Mike