Tweak width of differential-panel to match aphront-panels on differential

Summary:
The differential panels at the top of the differential revision view page
were 2px smaller than the divs on the bottom of the page (everything below
the table of contents). This diff makes differential-panel 2px wider so it
matches.

Test Plan: viewed a differential revision and checked that the divs lined up

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: 887