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:

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/6581eea069cd8e485b7fa4187ed4c043/image.png)

Closes #2926 

See merge request !1601