more fixes to fluid diff

Summary: always render "copy" column for text. this gives us consistent left alignment. also tweak css to be less wide for the copy and code coverage columns.

Test Plan: http://phabricator.dev/rP1a3bf098ae4ab4f8add4af744a6b93a257851fb0 looks even better

Reviewers: vrana, epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2005

Differential Revision: https://secure.phabricator.com/D4110