Add a preference to disable ghost comments

Summary: Ref T7447. At least some users dislike this feature so strongly that they'd prefer not to have it at all.

Test Plan: Viewed ghosts; toggled preference, no more ghosts.

Reviewers: chad

Reviewed By: chad

Subscribers: yelirekim, epriestley

Maniphest Tasks: T7447

Differential Revision: https://secure.phabricator.com/D12704

src/applications/differential/query/DifferentialInlineCommentQuery.php

@@ -148,6 +148,13 @@ final class DifferentialInlineCommentQuery
     assert_instances_of($new, 'DifferentialChangeset');
 
     $viewer = $this->getViewer();
+
+    $pref = $viewer->loadPreferences()->getPreference(
+      PhabricatorUserPreferences::PREFERENCE_DIFF_GHOSTS);
+    if ($pref == 'disabled') {
+      return $inlines;
+    }
+
     $all = array_merge($old, $new);
 
     $changeset_ids = mpull($inlines, 'getChangesetID');