Unbreak regex filename search

Summary:
D9087 adds a nice typeahead but breaks the existing regex
search by quoting the pattern. Ideally, this change won't break the
typeahead, which as far as I can tell doesn't use the `pattern`
argument.

Test Plan:
Not yet.
RFC as to whether this change makes sense, will fix my local setup and resend if so.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15500

src/applications/diffusion/conduit/DiffusionQueryPathsConduitAPIMethod.php

@@ -79,7 +79,8 @@ final class DiffusionQueryPathsConduitAPIMethod
     $offset = (int)$request->getValue('offset');
 
     if (strlen($pattern)) {
-      $pattern = '/'.preg_quote($pattern, '/').'/';
+      // Add delimiters to the regex pattern.
+      $pattern = '('.$pattern.')';
     }
 
     $results = array();