Make gitk use --topo-order instead of --merge-order

Make gitk use --topo-order instead of --merge-order

It's cheaper to calculate, and doesn't give different results depending
on the order of the arguments passed in (and is thus more appropriate
for something like gitk that can validly take the unordered "--all" flag
to show all branches).

The previous dup fix seems to have fixed --topo-order.  Holler if you
still see problems.