That was my first thought, too. But the input is not JSON. The input is an object graph. JSON stands for "JS Object Notation". Any graph with nodes that match the shape that the example JSON parses to is a valid input. (Though it doesn't help that the author refers to the graph as "json".) Still, getting that object graph is the other half of the owl.
It's been a while I haven't work on this, but if you do 'git2graph -r' within a repository folder, I believe it will generate the needed json for that repository.