ArcanistBundleTestCase: fix support of non-English environments

Summary:
Recently this unit test was always failing for some specific users.
I was able to reproduce the issue with an Italian unix environment:

    arc unit src/parser/__tests__/ArcanistBundleTestCase.php

Obtaining:

    counterexample
    EXCEPTION (Exception): Diff Parse Exception: Expected '\ No newline at end of file'.

After this change, the involved unit test always work also in my Italian environment.

Note that the `LC_ALL=C` means that all localization should be deactivated,
and we should instead use C-sourced strings (not translated in any language). Nice!

Closes T15927

Test Plan: Run the unit test. It does not fail anymore if you are Italian of French or whatever.

Reviewers: O1 Blessed Committers, mainframe98, 20after4

Reviewed By: O1 Blessed Committers, mainframe98, 20after4

Subscribers: mainframe98, aklapper, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15927

Differential Revision: https://we.phorge.it/D25809

src/parser/__tests__/ArcanistBundleTestCase.php

@@ -11,8 +11,10 @@ final class ArcanistBundleTestCase extends PhutilTestCase {
   }
 
   private function loadDiff($old, $new) {
+    // This unit test must parse 'diff' output in a language independent way,
+    // so we need LC_ALL=C.
     list($err, $stdout) = exec_manual(
-      'diff --unified=65535 --label %s --label %s -- %s %s',
+      'LC_ALL=C diff --unified=65535 --label %s --label %s -- %s %s',
       'file 9999-99-99',
       'file 9999-99-99',
       $this->getResourcePath($old),