WIP: handle updated merge requests
This handles the case where someone:
- makes a merge request
- later updates the description or title of the merge request to include a bug trailer
This seems likely to be a common workflow.
As written, I think it doesn't currently do anything if a bug trailer is removed. It also doesn't do anything with the last commit message (in contrast to when a MR is created, etc.)
Bug: T324150 Bug: T325116