How to squash commits in a GitHub pull request

So you’ve contributed some code to an open source project, say, Rails. And they’d like you to squash all of the commits in your pull request. But you’re not a git wizard; how do you make this happen?

Normally, you’d do something like this. I’m assuming upstream is a git remote that is pointing at the official project repository, and that your changes are in your 'omgpull' branch:

$ git fetch upstream
$ git checkout omgpull 
$ git rebase -i upstream/master
< choose squash for all of your commits, except the first one >
< Edit the commit message to make sense, and describe all your changes >
$ git push origin omgpull -f

GitHub will then automatically update your pull request with the new commits. Super easy!