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