Add colgroup in differential "shield" tables
Summary: Fixes minor brokenness from D8686.
Test Plan: Expanded a generated file, saw a normal-looking diff.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D8693