as found here:
https://bugs.launchpad.net/openstack-ci/+bug/929723
and
http://code.google.com/p/gerrit/issues/detail?id=2785

The changeMerged event provides the merged patchset information of a review/change that was merged to its destination branch.

Can this event be extended, or a new event created to include the SHA of the merge commit that was written/exists on the destination branch so that an event listener, like the Jenkins Gerrit Trigger Plugin, could fetch the specified branch and rewind to the specific SHA for a per-review-on-the-branch build trigger?

The refUpdated event has essentially a rev-list of individual commits with no indication of which were added as a result of review merge