> All we need is GitHub to support a `Depends on: #123` annotation which would hide commits already in #123 and not let you merge until #123 is merged.
You can get an approximation by having the PR target the branch used by #123.
This doesn't work across forks, though, and therefore doesn't compose with GitHub's model where you push your changes to your private fork and then open a PR from there.
There's also no native UI support for it in GitHub -- I'd expect to have a navigation element for stacked PRs like in Gerrit.
This means multiple stacked branches that you need to maintain on your own, doesn't it? That's the annoying part IMO, even when a short script would do or the rebases take a few characters in some efficient git UI.