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