Exercises to solidify Git mental models

[illicitonion]

We commonly see trainees making PRs that contain the commits of other PRs. Trainees don't seem to have a good mental model of branching. We should add some targeted exercise (possibly auto-marked by a script) to build up this practice.

This may just be as simple as having them create two PRs against the same repo from different branches, e.g. in CodeYourFuture/Module-Onboarding#67 and/or CodeYourFuture/Module-Onboarding#68