make_sort_from_pathkeys()'s method for choosing which of several

equivalent sort expressions to use was broken: you can't just look
at the relation membership, you have to actually grovel over the
individual Vars in each expression.  I think this did work when it
was written, but it was broken by subsequent optimizations that made
join relations not propagate every single input variable upward.
Must find the Var that got propagated, not choose one at random.
Per bug report from Daniel O'Neill.