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.
Reviewed By: btrahan
Maniphest Tasks: T5644
Differential Revision: https://secure.phabricator.com/D12147