12_repos.ipynb: Add more repositories, and a visualization

Add a few more large or large-ish example repositories, following the
draft of "Graph operations in Git version control system" slides and the
GitHub Blog article "Git clone: a data-driven study on cloning
behaviors":

 - LLVM repository
 - GCC repository
 - Homebrew repository
 - Ruby on Rails repository
 - jQuery repository

To visualize sizes of commit graphs extracted from those repositories,
plot number of nodes (vertices) versus the number of edges.

Author: jnareb