Scope syntax css rules to direct descendants only in diffs

Summary: Fixes T11641. We're overbroad here (and this may need more scoping?) but this seems to resolve the immediate issue.

Test Plan: Upload a few diffs and ask disabled accounts to comment on them inline.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T11641

Differential Revision: https://secure.phabricator.com/D17565