Fix "reply" to left-side inline appearing on right side

Summary: Fixes T8098. This parameter was being misinterpreted over the wire.

Test Plan: Replied to left-side inline, got left-side reply.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8098

Differential Revision: https://secure.phabricator.com/D12747