R1:84fd5cd5bb4e

Fix an issue where intracontent empty lines were incorrectly trimmed in quoted blocks

Summary: Fixes T13335. When processing quoted blocks, we remove leading empty lines. This logic incorrectly continued after encountering a nonempty line.

Test Plan: Added a test, made it pass. Previewed blocks in web UI.

Maniphest Tasks: T13335

Differential Revision: https://secure.phabricator.com/D20965