Add link on commit page to merge request that introduced that commit
We can now do this relatively easily, since every MR is copied to
refs/merge-requests/:iid/head, and we already have the list of branches in which a commit occurs. We can see if any refs like that are in that list, parse the IID out of there, look up the MR by IID, and link to it.
uses the icon from #29584 (closed)
Note: This however needs backend support as described #2383 (comment 26341393)