Automated check for commits on branches marked as integrated

Every now and again a vigilant tester spots that an issue hasn't been integrated into a branch properly.

e.g. I made a mistake on MDL-44258 and the commits were completely missing from the 25 branch.

Usually this should always be discovered by testing, but it would be nice to have a backstop. We could have a CI check which looks for integrated issues and git greps for the issue number. It could take action on issues without git grep results.

• It would have false positives occasionally, so we'd need a way to ignore it. Although there is discussion elsewhere of a pre-checker check for issue numbers too. So it should be fairly good.
• It could simply mark as failed issues marked as integrated which do not have git grep results for the issue number.