Fix displaying of inlines related both to visible and hidden diff

Summary: I tried also filling the column by empty space but this looks better.

Test Plan: Displayed a comment both with visible and hidden inlines.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1216

Differential Revision: https://secure.phabricator.com/D2789