Level: Split changes and merge master into both while keeping them sequential

You have a branch that you've been developing on for a while.

- Start with two branches, master, intro.
- For intro, you build an api that you want to merge into master
- You want to merge all the intro changes in at once, but it turns out one level is going to be harder to develop than the others.
- Soft reset and split the changes into two commits (can also do interactive rebase, which is less destructive)
- You now have master, api, intro.
- You go to merge api into master, but you want to test it out first, so you merge master into api.
- It turns out there are merge conflicts, so you resolve them
- You want to resolve the conflict in the intro branch too, so you rebase intro on top of api