"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.

打开 Open tutorials/actually-using-git/lessons/conflicting-branches.md，并逐步按照教程操作。在每一步之后添加检查点，显示 git branch、git status 和 cat planets.md 的预期输出。当学习者能够在每一步之后比较其仓库状态，并识别出状态何时发生偏差时，即视为完成。

由索引模型根据 Issue 内容生成。