gitlab: update regex matching system notes about MR branch updates
GitLab upstream commit v8.16.0.pre~449^2
(Rephrase some system notes to be compatible with new system note style, 2016-11-23) changed the wording of the note that GitLab adds when a MR branch is updated. Furthermore, the "Compare with previous version" note is now singular. Revise our regex to match both the old and new forms of the note.
Fixes: #43 (closed)