Handle "hg pull" return code change between 2.1 and 2.1.1 more gracefully

Summary:
See https://github.com/facebook/phabricator/issues/102. Between Feb 1 and Mar 1, the hg released changed the exit code behavior of "hg pull". This broke us mildly (and a bunch of other applications more severely, which is why it was reverted).

Detect the common case of this (english) and don't fail.

Test Plan: @killermonk, can you try applying this? I'll try to do an upgrade to 2.1 and see if I can also do a proper test.

Reviewers: Makinde, btrahan, killermonk

Reviewed By: btrahan

CC: killermonk, aran, epriestley

Differential Revision: https://secure.phabricator.com/D1948