Fix an issue where the "%%%" parser could match too many lines in unterminated blocks

Summary: Fixes T13530. The block parser could match too many lines in an unterminated "%%%" literal block. Adjust the logic to stop doing this (and hopefully be a little easier to read).

Test Plan: Added a failing test, made it pass.

Maniphest Tasks: T13530

Differential Revision: https://secure.phabricator.com/D21208