Apply phutil_utf8ize() to stderr output from VCS commands prior to logging

Summary: Ref T10228. Commands like `git-http-backend` can emit errors with raw bytes in the output. Sanitize these if present so we can log them in JSON format.

Test Plan: Edited this into production. >_> sneaky sneaky <_<

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10228

Differential Revision: https://secure.phabricator.com/D15144

src/applications/diffusion/controller/DiffusionServeController.php

@@ -466,7 +466,10 @@ final class DiffusionServeController extends DiffusionController {
     if ($err) {
       return new PhabricatorVCSResponse(
         500,
-        pht('Error %d: %s', $err, $stderr));
+        pht(
+          'Error %d: %s',
+          $err,
+          phutil_utf8ize($stderr)));
     }
 
     return id(new DiffusionGitResponse())->setGitData($stdout);