Summary:
Includes a new Block in Herad emails which tells the user about which commits were merged in a merge commit
Otherwise the email would just say "Merge branch XYZ". Ref T8295
Test Plan: imported various commits (and merges) and watched resulting herald emails for all of them
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T8295
Differential Revision: https://secure.phabricator.com/D12993
