R1:a5e29f3ffa9a

Fix an ancient ad-hoc string truncation

Summary: Fixes T11139. We missed this years ago when we moved to PhutilUTF8StringTruncator.

Test Plan: {F1686072}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11139

Differential Revision: https://secure.phabricator.com/D16105