Ok this is nice. Of course, if you're just rebasing before reviewing and merging the PR into master it makes this pointless.
Odd that they never mention this in the post, I was under the assumption that rebasing before creating the PR is standard practice, at least that's how we do it at work and on all the projects I've contributed to.
EDIT: I've never used bitbucket, so maybe this is just a github community thing with the rebasing?