Honor "fixup!" commits in merge requests

When an admin selects "Fast-forward merge" Merge method in Project Settings, a "Rebase merge request" button is present in merge requests where the master (or the branch to which a merge is requested) has changed since submitting the merge request.

When one then pushes further commits to the merge request's source branch which address review comments in a way of doing git commit --fixup <sha> (e.g. not to mess up previous inline comments made in the diff by a reviewer, and/or to be clear about what is being changed), and then the reviewer chooses to use the "Rebase merge request" button, I would expect the "fixup! ..." commits to be squashed into the commits they are fixing, just like if git rebase --autosquash --interactive master was run with accepting the proposed TODO list for the rebase. But that is not the case (I just tried on a demo project) - the commits in the source branch are rebased on the latest master, but with the "fixup!" commits treated as completely ordinary commits (i.e. still present).

Proposal

If a merge request contains a fixup! commit that targets a commit in the feature branch, the merge request interface should handle this according to the merge method in the project settings:

• Merge commit
  • If Squash and Merge is enabled, the fixup commit doesn't matter because it'll be squashed away
  • Else, show a message and a rebase button
• Merge commit with semi-linear history - Rebase merge request button should rebase with --autosquash support
• Fast forward merge – Rebase merge request button should rebase with --autosquash support

Links / references

• https://robots.thoughtbot.com/autosquashing-git-commits