Noisy mentions from fork (useless "last updated" sorting)

Summary

If I push to my fork commits with references to the issue I see mentions noise in the original project issue. The problem is that we have to "remember if a commit SHA has already been mentioned already" (@godfat) .

This also breaks MR and Issue sorting by "last updated" (not actually breaks but makes it useless)

Also the discussion is available here https://gitlab.com/gitlab-org/gitlab-ce/issues/20527#note_15676024

Steps to reproduce

Just update your fork and issue will come up

Expected behavior

Every commit should notify origin issue only once

Actual behavior

We have a lot of mentions from the same commit from every fork

Relevant logs and/or screenshots

https://gitlab.com/gitlab-org/gitlab-ce/issues/20527#note_15676024

/cc @DouweM @rspeicher