merge-requests against non-"parent" repositories

Description

It seems like it is currently only possible to submit a "merge requests" against the one repository one has forked from (the "parent" repository, or - in gitlab lingo - the "repository we have a fork relationship with"

Esp. it is impossible to:

• merge request against a "child" repository (a repo cloned from ours)
• merge request against a "sibling" repository (a repo that - like ours - has been cloned from a common ancestor)
• merge request against a "grand-parent" repository

Use case: non-hierarchical merging between projects. Not all projects have an authoritative master repo from which you can clone and which accepts MRs.

Proposal

I would like to see a way to exchange MRs between any two repositories that share a common history. (no matter how there fork relationship is; even if they have both been cloned from an outside-gitlab repository and uploaded independently).

The search for related (sharing-a-common-history) repository is potentially very time consuming (it involves scanning all repositories on the server). this work should therefore be left to the user (the project admin), who should be able to request the check for a specific repository.

Links / references

I'm pretty sure that I already proposed this before, but cannot find it anymore.