Improved attribution when squashing commits

In 2016, GitHub introduced commit squashing when merging a pull request. Then in 2018, we added support for commit co-authors. Today, we’re combining these features to improve the squash-and-merge experience.

Before today, whoever opened the pull request became the sole author of the squash commit. Now, we will automatically credit every commit author in the pull request as a co-author on the squash commit.