Improve display of "Context not available" in 1-up view

Summary: Ref T2009. Makes "Context not available" render and behave correctly.

Test Plan: Viewed diff with missing context in 1-up view, looked marginally better.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D11973