Don't syntax highlight codebase pattern search results

Summary:
Ref T5644. Ref T7472. Currently, we highlight each line of pattern search results in Diffusion.

• This is incredibly slow for non-PHP languages which need to shell out to Pygments.
• A lot of this highlighting isn't very useful anyway, because it doesn't have any context.

Instead, try to highlight pattern matches but don't highlight the source itself.

Test Plan: {F349637}

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7472, T5644

Differential Revision: https://secure.phabricator.com/D12141