Fully detect uninteresting commits.

Fully detect uninteresting commits.

With the change in the previous round, we are guaranteed to come up
with the list of all relevant merge bases, but sometimes we do not
fully mark unintersting ones due to a horizon effect.  Add a phase to
postprocess, so that we mark all ancestor of "interesting" commit.

This also changes the default ordering of shown commits back to
chronological order, and adds --topo-order flag to show them in
topological order.

Signed-off-by: Junio C Hamano <junkio@cox.net>