Beisdes analyzing properties of the commit graph, which doesn't seem to be a common task, what about them is so useful that they need to be stored in commit headers now? For instance, what day-to-day operations do they actually make faster? At first, it sounded like it might be for easier navigability (e.g. expanding git-rev-parse) but I don't see that in this description.
"git tag --contains" needs to know the generation number of old tags, which is expensive to calculate. "git describe" would be faster for the same reason.
