A week ago, I was at LFNW and as I was talking with Larry Hastings after his talk, he said (paraphrased):
There's something in Git that makes the Pull Request workflow possible that isn't there in Mercurial. That's why Pull Requests on BitBucket are not great.
(For context, we were discussing the fact that Python is moving from Mercurial to Git because of the PR workflow, as opposed to the file-bug-then-attach-patch workflow.)
What is he talking about here? Neither of us could come up with the name of whatever it was. I've searched the web with no luck.