Dispatch event after collecting changes in diff

Summary:
FB currently starts Sandcastle push before starting the diff workflow.
If `arc diff` commits something then we need to restart the push.
I want to avoid this by starting the push after commit.

Test Plan: Will test after implementing the listener.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4785