Add a new MergeValidationListener interface which can be used to
provide additional validation of commits before they are merged to
the git repository.
Add a MergeValidators class to invoke the merge validator listeners,
and call it from MergeOp before the commit is merged.
Add a listener to invoke validation provided by plugins.
Change-Id: I325d923f5cc0245b60e86a035329b640c1682d48
8f9e9f58e2