Git has lots of tools to map the history of a git repository. But, I have not been able to find tools to map networks of git repositories.
Possibly noone has done it because such networks tend to be pretty centralized really, with low levels of (visible) interconnection, and so not that interesting, like this network of all known branches of ikiwiki.
Soon after I started using git-annex, I realized the repositories using it would tend to be more interconnected, and less centralized, and generally a more complex, rich fabric than I'd seen in my use of git before.
So I've built "git annex map", which uses graphviz to spit out fun maps like this.
(Green nodes are trusted drives; red nodes are not currently available.)
Previously: git tube map