Allow system agents in commit message object lists

Summary:
When system agent adds a comment then he is added to CC.
When I amend and update then I get message "Commit message references nonexistent ..."

Test Plan: Update revision with system agent in CC.

Reviewers: epriestley

Reviewed By: epriestley

CC: michalburger1, aran

Differential Revision: https://secure.phabricator.com/D2100

src/applications/differential/field/specification/base/DifferentialFieldSpecification.php

@@ -658,8 +658,7 @@ abstract class DifferentialFieldSpecification {
     $object_map = array();
 
     $users = id(new PhabricatorUser())->loadAllWhere(
-      '((username IN (%Ls)) OR (email IN (%Ls)))
-        AND isSystemAgent = 0',
+      '(username IN (%Ls)) OR (email IN (%Ls))',
       $value,
       $value);