WIP: Try to enable merge request pipelines

What does this MR do?

Picking:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27116
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27282

Tests:

• pipeline with no changes: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27387
• pipeline with changing docs only: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27388
• pipeline with changing rspec only: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27389

This tries to enable merge-request pipelines for CE/EE.

Related https://gitlab.com/gitlab-org/gitlab-ce/issues/57190

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Style guides