Analyzing rescopes using git diff/git patch-id shouldn't happen directly on event threads

[Bryan Turner]

It may be desirable to move all of the event listeners to a background ExecutorService but, at a minimum, AutoUnapproveListener.onPullRequestRescoped should delegate to an executor because the git diff and git patch-ids commands it runs may be slow. That can lead to event threads backing up and events being dropped, which can result in difficult to diagnose runtime issues across the system.

[Bryan Turner]

Pull request #29 included changes to use the executor for calculating whether to remove approvals after a rescope.