Disallow /source/ in robots.txt

Summary: Ref T4245. We disallow `/diffusion/` in robots.txt already because indexers tend to get lost blaming every line of every file throughout history, but didn't update the list for the `/source/` alias. Update it.

Test Plan: Visited `/robots.txt` locally.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D17637

src/applications/system/controller/PhabricatorRobotsController.php

@@ -20,6 +20,7 @@ final class PhabricatorRobotsController extends PhabricatorController {
 
     $out[] = 'User-Agent: *';
     $out[] = 'Disallow: /diffusion/';
+    $out[] = 'Disallow: /source/';
 
     // Add a small crawl delay (number of seconds between requests) for spiders
     // which respect it. The intent here is to prevent spiders from affecting