R1:84d0a6ac2de0

Simplify Differential field selector

Summary:
If I use my own selector then it doesn't respect Phabricator config.

Also I hated this method.

Test Plan: Used default selector, displayed revision.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3307