- The usage of this in `brew contributions` wasn't correct for a user
with 5 authored commits to homebrew/cask that had been committed by
other people, the numbers would turn out as 5 authored, 5 committed.
- I decided to do this properly by getting the SHAs for author and
committer and determine the differences between the two arrays.
This also accounts for when authored commits are 0, or committed
commits, or both.
- Add tests, because I don't want to fix this a third time!