GitLab B.V. CEO here. The proposed diff is interesting and I love the way they explained the idea. During the development of GitLab (over 10.000 commits) we never experienced the problem that was mentioned. Maybe it is because our codebase contains many tests. One related idea that a GitLab user proposed is not testing the feature branch but testing the merge of the feature branch into master. This would require retesting of all feature branches on every commit to master. We're open to proposals to add this to GitLab CI. But so far we didn't experience this problem frequently.