Changes to git revert/branch session

- Add refresher of git add/commit etc from the first day
- Exercise using `git branch` relied on testing discussion ([see Issue 88](https://github.com/UW-Madison-ACI/boot-camps/issues/88)); instead have students implement a variance function.
- **Note**: still need caution about location of the students' `stats.py` file (will they have cloned the boot-camps repository?