Changes graph() to colorededges(), which operates on the new
generic DAG walks and adds color and edge information needed
by the web graph.
This is in preparation of adding DAG walk filters, like the
linear run collapser in the next patch. The idea is to have
a bunch of changelog walkers that return basic data. Then we
can filter this data. Finally we add edge and formatting info
suitable for the output media we want to target (glog, hgweb).
Semantically, it is better to use None over any other value when there is
"no value". Using maxint in this context is quite hackish, and is not forward
compatible.
