Add very basic conflict detection to Phriction

Summary: Ref T4768. This is extremely basic, but will stop conflicts from going silently unnoticed.

Test Plan: {F156219}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4768

Differential Revision: https://secure.phabricator.com/D9167