Exercises to solidify Git mental models

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 https://github.com/CodeYourFuture/Module-Onboarding/issues/67 and/or https://github.com/CodeYourFuture/Module-Onboarding/issues/68