Don't invoke pygmentize to highlight "txt" or "text" files

Summary: Ref T5644. Caught one more of these: we run pygmentize even when we know the file won't be highlighted.

Test Plan: Checked that we don't try to shell out to highlight text files.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5644

Differential Revision: https://secure.phabricator.com/D12147