"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.

Open tutorials/actually-using-git/lessons/conflicting-branches.md and follow the lesson step by step. Add checkpoints after each step showing the expected output of git branch, git status, and cat planets.md. Done means learners can compare their repository state after every step and identify when it diverges.

Written by the indexing model from the issue text.