Pass HGPLAIN in the environment when testing "hg" version
See T12972. In Phabricator, we always execute with HGPLAIN but this didn't make it into the standalone binary analyzer:
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.
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D18610