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.