When the HEAD of a project is updated Gerrit now informs about this by
sending an event. Plugins can implement a listener to be notified
about this event. Having this new event enables the replication plugin
to replicate HEAD updates.
Change-Id: I4dd7969020880190668190e8345197a23dbc92f1
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>