Add a system note and update relevant merge requests when a branch is deleted or re-added
If a branch is deleted with an open merge request, amended offline, and then pushed again,
GitLab doesn't bother to update the merge request even though the last commit ID and/or
code may have changed. This MR ensures that each push will update any relevant merge
requests and adds a system note if this happens as well.
The new messages look like:

Closes#2926
See merge request !1601