The listener interface is currently only used by MergeOp, which is REPO_BEFORE_DB. Reasoning about the order in which listeners are called is going to be more difficult if we fuse the repo and change meta ref update phases; sidestep the problem by just removing support for listeners in the odd DB_BEFORE_REPO case. Change-Id: Ia2a1c1f827247602eab1f8527f3cd3118f740f23