Pass HGPLAIN in the environment when testing "hg" version

Summary:
See T12972. In Phabricator, we always execute with HGPLAIN but this didn't make it into the standalone binary analyzer:

https://secure.phabricator.com/source/phabricator/browse/master/src/applications/diffusion/protocol/DiffusionMercurialCommandEngine.php;c71cb944a42851de00b2129f15899f7681b0f7ed$31

HGPLAIN disables translations and extensions which may make hg version do something unexpected.

Test Plan: Verified my local version of Mercurial is still identified correctly in Config. I have yet to figure out how to install non-English Mercurial so I didn't truly test this, but our use of HGPLAIN in other contexts is well-established.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18610