Show repository in Differential emails

Summary: Ref T5137. Listing the repository in Differential emails makes it easy to filter.

Test Plan: Eye-ball it.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: young_hwi, epriestley, Korvin

Maniphest Tasks: T5137

Differential Revision: https://secure.phabricator.com/D9609