As an expansion on that, as far as I know there's only one implementation of Fossil. There was at least a Java one for git, I believe, and I don't think upstream git is related to libgit2 either. And GitHub is doing… something not visible from the outside.
(… looking at the parent, though, I imagine there might be some information from the inside…)
(… looking at the parent, though, I imagine there might be some information from the inside…)