graphlog: add a way to test the 'groupbranchiter' function We add an experimental config option to use the topological sorting. I first tried to hook the 'groupbranchiter' function in the 'sort' revset but this was useless because graphlog enforces revision number sorting :( As the goal is to advance on the topological iteration logic, I see this experimental option as a good way to move forward. We have to use turn the iterator into a list because the graphlog is apparently not ready for pure iterator input yet.