Updating an existing Github pull request

Akka uses single commits for its pull requests, so that changes can be easily understood and rolled back. Pull requests are contained in their own branch. Here's how I updated my single-commit pull request with some changes and resubmitted it.

1. Rebase the last two commits interactively.

   git rebase -i HEAD~2
   

2. When asked, squash the latest commit into the previous one.
3. Replace the existing upstream branch with the updated one. Use + to signify that the updated branch isn't based on the old one. (Often origin is used instead of upstream.)

   git push upstream +wip-2798-ensuring-rich