Merge requests `diffs.json` shows high Gitaly request rates (Projects::MergeRequests::DiffsController)

At https://log.gitlab.net/app/kibana#/discover?_g=h@1f9dccc&_a=h@886529b (internal) we're able to see Gitaly RPCs are being called in a very high rate in a few MRs outside gitlab-org (if we filter further down the json.path: *gitlab* we won't see such high rate).

Interestingly, the highest rates are coming from the same project (which is private).

I can't look further given I don't have access to these MRs (maybe you could show us some calls @stanhu?).