Handle fixup commits

[aschmolck]

Marge should probably squash fixup commits automatically.

[jcpetruzza]

The problem here is going to be this GitLab feature: every time you push a fixup! commit, GitLab marks the merge request as WIP. So that by the time you assign it to marge, she will refuse to merge it since it is a WIP branch.

In principle marge-bot could try to reset the WIP status, but in general I'd feel uncomfortable with this. Ideally GitLab should automatically unset the WIP status once all the commits that lead to the WIP status are gone. Then, just merging the rebased version will work.

[aschmolck]

Gitlab setting wip status in the presence of fixups is maybe understandable (although, why not just auto-fix them on merge?), but if the status persists beyond the existence of the fixup commits themselves (as I assume you found out on testing), we hopefully can convince them to change that. I'm comfortable resetting WIP status if we gate the whole auto-squashing thing behind a feature flag though (and make it very clear that this entails resetting WIP status).