Yes, that's why I asked it as a response to them saying GitHub was just fine :) With Git, I work around it by adding `--fixup` commits and then doing a final `git rebase --autosquash` just before landing it, but it would be nice to have a better workflow.
(The mention of the GitHub SVP being interested in stacked diffs sounds good in that regard. I'm also keeping an eye on Tangled and, now, on ERSC.)
Patch-based workflows in general (where you heavily use git rebase -i to curate a smaller set of "perfect" commits for anything you'd PR, rather than just piling new commits onto every PR) don't work well with the GitHub approach, which heavily favors merging stuff through the web interface (making merge or squash commits along the way).
You can make it work of course, but GitHub tends to behave in weird ways when it comes to how it's interface works with patch-based workflows. Perhaps a better estimate would be to see how it compares to a forge like Phorge or Sourcehut.