Fix table layout bug for inline comments

Summary:
The filename header for inline comments used to span 2 columns - the line number
and the comment. With the addition of a column for the diff (to link to inline
comments on previous diffs), the filename header should now span 3 columns
instead of just the line number and diff, leaving the comment squished to the
right.

Test Plan:
Opened a differential revision with an inline comment from a previous diff, and
saw that the filename header continued across the comment. Also checked an
inline comment on a current diff, and saw that it looks fine.

Reviewers: epriestley, btrahan, jungejason

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1340