show merged commits in herald emails for merge commits

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