In Remarkup, prevent "%%%" on a line from self-terminating the literal block

Summary:
Fixes T12621. Currently, %%% on a line by itself can both start and terminate a literal block. This is silly and presumably not what anyone ever intends.

Instead, if %%% started a block, don't let it terminate the block.

Test Plan:

• Added a failing test.
• Made it pass.

{F4920751}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12621

Differential Revision: https://secure.phabricator.com/D17772