Use HGPLAIN for local hg commands in Phabricator

Summary: See D1707 -- just in case the Phabricator server is configured
suspiciously.

Test Plan: Cursory inspection.

Reviewers: Makinde, btrahan

Reviewed By: Makinde

CC: aran, epriestley

Maniphest Tasks: T922

Differential Revision: https://secure.phabricator.com/D1708