Fix Diffusion commands in non-English environments

Summary:
Closes T15966

Force Diffusion commands to be executed in the "LC_ALL=C" language mode, that is, English,
defusing whatever custom language you have in your LANGUAGE variable.

Related reading:

https://we.phorge.it/T15872

Test Plan:
Make sure Subversion is installed on your system. Explicitly set `LC_ALL`.
Finally, this unit test works, even if you have something esoteric in your LANGUAGE:

    arc unit ./src/applications/repository/worker/__tests__/PhabricatorChangeParserTestCase.php

Before, it was crashing, like mentioned in T15966.

Reviewers: O1 Blessed Committers, aklapper

Reviewed By: O1 Blessed Committers, aklapper

Subscribers: aklapper, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15966

Differential Revision: https://we.phorge.it/D25846

src/applications/diffusion/protocol/DiffusionCommandEngine.php

@@ -175,10 +175,12 @@ abstract class DiffusionCommandEngine extends Phobject {
     $repository = $this->getRepository();
 
     $env = array();
-      // NOTE: Force the language to "en_US.UTF-8", which overrides locale
+      // NOTE: Force the language to "C", which overrides locale
       // settings. This makes stuff print in English instead of, e.g., French,
       // so we can parse the output of some commands, error messages, etc.
-    $env['LANG'] = 'en_US.UTF-8';
+      // Note that LANG can be ignored if there is LANGUAGE.
+      // https://we.phorge.it/T15872
+    $env['LC_ALL'] = 'C';
 
       // Propagate PHABRICATOR_ENV explicitly. For discussion, see T4155.
     $env['PHABRICATOR_ENV'] = PhabricatorEnv::getSelectedEnvironmentName();