groubranchhiter: indent most of the inner code We are going to add an additional layer of indentation to support non-contiguous revset. We do it in a pure code movement changeset to help the readability of the next changeset.

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.

graphmod: add a function for topological iteration This changeset introduces a function to perform topological (one branch after the other) iteration over a set of changesets. This first version has a lot of limitations, but the approach should be flexible enough to allow many improvements in the future. This changeset aims to set the first stone more than providing a complete solution. The algorithm does not need to know the whole set of nodes involved before emitting revision. This makes it a good candidate for usage in place like `hg log` or graphical tools that need a fast first result time.