Deduplicating duplicated field values for Commit Messages / Differential

Summary: Especially when doing 'arc diff' containing multiple commits which have pre-filled template fields. Names would pop up multiple times

Test Plan:
{F36314}

Local `arc diff` with some uber-branch also agrees with me

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5371