Native mode: Merge Request IID payloads and make sure we never prune the default GitLab branch
Two different topics in one
- Merge Request IID is needed to allow people to merge in the Rails app on protected branches (see heptapod#368 (closed)). This was done in the stable branch and will also publish it there, but it is unlikely it will be released as Hepapod 0.16.3
- For the native mode, we must make sure the default GitLab branch is never pruned. In turn that means that in pruning due to shadowing by bookmarks, we must not even schedule it.