Fix an issue where lines with more than one pattern match highlighted improperly in Diffusion

Summary:
Ref T13339. If a search pattern matches more than once on a line, we currently render the line incorreclty, duplicating some of the text.

`substr()` is being called as though the third parameter was `end_offset`, but it's actually `length`. Correct the parameter.

Test Plan:
Before:

{F6676625}

After:

{F6676623}

Maniphest Tasks: T13339

Differential Revision: https://secure.phabricator.com/D20695

src/applications/diffusion/view/DiffusionPatternSearchView.php

@@ -47,7 +47,7 @@ final class DiffusionPatternSearchView extends DiffusionView {
           $offset = $match[1];
           if ($cursor != $offset) {
             $output[] = array(
-              'text' => substr($string, $cursor, $offset),
+              'text' => substr($string, $cursor, ($offset - $cursor)),
               'highlight' => false,
             );
           }