Fix odd line number line wrapping on embedded pastes ({Pxxx})

Summary: Ref T13126. After SourceView changes, embedded pastes with the {Pxxx} syntax are line-wrapping line numbers in Safari, at least. Put a stop to this.

Test Plan: Viewed a {Pxxx} with more than 10 lines. Before: weird line wrapping; after: nice consistent display.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13126

Differential Revision: https://secure.phabricator.com/D19393