From tickets to merge requests and vice versa

Ben, Matthew I wrote up notes on how to get from MR to issue, and from issue to MR, in my help page: https://gitlab.haskell.org/ghc/ghc/wikis/working-conventions/git-lab-spj (See "Issues" and "Merge requests" headings) Alas, it seems that I was lying * Issue #16411 does not have a "relevant merge request" link to MR!520, even though the latter plainly mentions the former. That seems deeply strange, and makes it hard to see if the issue has a MR, let alone which one it is. * Somehow MR!520 says "closes #16411" rather than just "mentions #16411". Perhaps this is good, but how did it know to do that? Simon