Support Mercurial pretxnchangegroup hooks

Summary: Ref T4189. Fixes T2066. Mercurial has a //lot// of hooks so I'm not 100% sure this is all we need to install (we may need separate hooks for tags/bookmarks) but it should cover most of what we're after at least.

Test Plan:
  - `bin/repository pull`'d a Mercurial repo and got a hook install.
  - Pushed to a Mercurial repository over SSH and HTTP, with good/bad hooks. Saw hooks fire.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2066, T4189

Differential Revision: https://secure.phabricator.com/D7685

src/applications/diffusion/ssh/DiffusionSSHMercurialServeWorkflow.php

@@ -42,7 +42,8 @@ final class DiffusionSSHMercurialServeWorkflow
     $command = csprintf('hg -R %s serve --stdio', $repository->getLocalPath());
     $command = PhabricatorDaemon::sudoCommandAsDaemonUser($command);
 
-    $future = new ExecFuture('%C', $command);
+    $future = id(new ExecFuture('%C', $command))
+      ->setEnv($this->getEnvironment());
 
     $io_channel = $this->getIOChannel();
     $protocol_channel = new DiffusionSSHMercurialWireClientProtocolChannel(