Diffusion - move some DiffusionRequest queries to occur over Conduit

Summary: Ref T2784. This one was a wee bit complicated. Had to add PhabricatorUser and concept of initFromConduit (or not) to DiffusionRequest.

Test Plan: foreach repo, visited CALLSIGN and clicked a commit and verified they laoded correctly. Hacked code to hit NOT via Conduit and repeated tests to great success.

Reviewers: epriestley

Reviewed By: epriestley

CC: chad, aran, Korvin

Maniphest Tasks: T2784

Differential Revision: https://secure.phabricator.com/D5928

src/applications/diffusion/exception/DiffusionExpandCommitQueryException.php

@@ -0,0 +1,21 @@
+<?php
+
+final class DiffusionExpandCommitQueryException extends Exception {
+  const CODE_UNPARSEABLE = 'unparseable';
+  const CODE_MISSING = 'missing';
+  const CODE_INVALID = 'invalid';
+
+  private $statusCode;
+  public function getStatusCode() {
+    return $this->statusCode;
+  }
+
+  public function __construct($status_code /* ... */) {
+    $args = func_get_args();
+    $this->statusCode = $args[0];
+
+    $args = array_slice($args, 1);
+    call_user_func_array(array('parent', '__construct'), $args);
+  }
+
+}