Lift inline comment state transactions into core (in Differential)

Summary: Ref T1460. Follows D12129 and reduces code duplication.

Test Plan: Changed inline state in Differential.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1460

Differential Revision: https://secure.phabricator.com/D12130