Summary:
When the user runs "arc diff --create" or triggers it via "arc diff
--auto", prefill the template as best we can.
Test Plan:
Ran "arc diff --auto" with a template commit message in the working
copy under various configurations, results seemed reasonable.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran, epriestley, dmi
Maniphest Tasks: T614
Differential Revision: https://secure.phabricator.com/D1719