Pass the constructed message to the TYPE_DIFF_DIDBUILDMESSAGE event.

Summary: We want to use TYPE_DIFF_DIDBUILDMESSAGE to abort arc diff when a message doesn't fit some

Test Plan: Created an EventListener subscribed to TYPE_DIFF_DIDBUILDMESSAGE, validated the 'message' field was filled in

Reviewers: vrana, epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D4361