Avoid duplicate pipelines in merge requests

Problem to solve

When only:merge_requests is used in CI configuration, pushes to a MR create two pipelines: a "normal" one, and a a "merge request" one.

Target audience

Further details

Well, maybe only:branches is needed in addition to only:merge_requests. What I mean is that one may want different jobs run for pushes and for merge requests, but with the former a subset of the latter. In this situation, one does not want two pipelines being created in a merge request, one suffices. This may be unavoidable (or with difficulty) when creating a MR, since the push pipeline was already created, but for later pushes it should be easy to detect that a MR exists.

This was already discussed in #15310 (closed), but it seems it was postponed... and I don't want it to be forgotten.

Proposal

A way to avoid creating a "branch" pipeline when pushing to a branch, if a "merge request" pipeline is created.

Maybe the easiest is a project setting, similar to auto-cancel pipelines.

What does success look like, and how can we measure that?

Fewer unneeded pipelines created, less resource waste, shorter queueing times.