Before executing svnserve, change the CWD to a readable directory

Summary: Fixes T10941. This avoids a confusing dead end when configuring Subversion hosting, where `svnserve` will fail to execute hooks if the CWD isn't readable by the vcs-user.

Test Plan:
  - Updated and committed in a hosted SVN repository.
  - Ran some git operations, too.
  - @dpotter confirmed this locally in T10941.

Reviewers: chad

Reviewed By: chad

Subscribers: dpotter

Maniphest Tasks: T10941

Differential Revision: https://secure.phabricator.com/D15879

src/applications/diffusion/protocol/DiffusionGitCommandEngine.php

@@ -24,8 +24,7 @@ final class DiffusionGitCommandEngine
     // really silly, but seems like the least damaging approach to
     // mitigating the issue.
 
-    $root = dirname(phutil_get_library_root('phabricator'));
-    $env['HOME'] = $root.'/support/empty/';
+    $env['HOME'] = PhabricatorEnv::getEmptyCWD();
 
     if ($this->isAnySSHProtocol()) {
       $env['GIT_SSH'] = $this->getSSHWrapper();