Creating a branch doesn't check it out by default. To do both at once, it's either "checkout -b" or "branch -c", I forget which because it's "b c" in Magit.

It's still on master because `git branch` just creates and doesn't do checkout.

(Which is why I've almost exclusively switched to `git checkout -b <newbranch>` for creating. How often do I want to create but not switch to the branch?)