"Actually Using Git": create checkpoints along the way

In `tutorials/actually-using-git/lessons/conflicting-branches.md`, folks can gloss over small steps and get their repo into an incorrect status. They then go through several steps before they discover this, and don't realize why.

We should provide them with "checkpoints" that show exactly what the output of `git branch`, `git status`, and `cat planets.md` should be after every step.