Alternative would be: clone the repo, do your changes, run gitk to be sure.
Which has advantage that result looks exactly the same as it will in production (no new diagram type b/c only using git and gitk), with zero risk that sim differs from reality.
Hehe jk you make a fair point, and in fact I do have a bunch of work left to do to make sure my simulations do match up with Git's behavior as closely as possible.
One big benefit I was going for with Git-Sim though is to interrupt the developer workflow as little as possible.
Changing directories, running a new clone (which could take a mildly annoying amount of time), and running gitk is a pretty big context-switch.