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