Swap "-R" and "serve" argument order for Mercurial

Summary: See <https://discourse.phabricator-community.org/t/unable-to-use-current-mercurial-on-debian-stretch/391/13>. I missed that `-R` is order-sensitive.

Test Plan: Verified both orders work on 3.5.2.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18616

src/applications/diffusion/controller/DiffusionServeController.php

@@ -769,7 +769,7 @@ final class DiffusionServeController extends DiffusionController {
     }
 
     $command = csprintf(
-      '%s serve -R %s --stdio',
+      '%s -R %s serve --stdio',
       $bin,
       $repository->getLocalPath());
     $command = PhabricatorDaemon::sudoCommandAsDaemonUser($command);