R1:33da9f833fdd

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