Fix an issue with TextAreaEditField affecting Paste

Summary: Fixes T10952. Fixes T10930. I didn't implement this method correctly when I expanded this field for repositories.

Test Plan: Edited a paste without warnings.

Reviewers: avivey, chad

Reviewed By: chad

Maniphest Tasks: T10930, T10952

Differential Revision: https://secure.phabricator.com/D15892