Notify user when fast-forward merge no longer possible

Problem to solve

When a merge request is set to "Merge when pipeline succeeds" but becomes no longer possible to merge due to fast-forward settings, there is currently nothing notifying the user that the MR was not merged. The user has to rely on going back to the MR and seeing the updated status.

Proposal

We should notify the user and add a todo when this happens.

Who should be notified?

• the person who selected "Merge when pipeline succeeds" probably
• maybe every assignee on the MR