Yell at developers if they don't replace the default revision title with thier new revision title.

Summary: Same as the title

Test Plan: Will test locally by running arc diff

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, AnhNhan, starruler

Maniphest Tasks: T2663

Differential Revision: https://secure.phabricator.com/D5297