Make sure READMEs can hit the markup cache in Diffusion

Summary: Ref T2683. Normally not a big deal, but if a readme has some codeblocks missing the cache can slow things down.

Test Plan:

• Verified we hit the cache.
• Verified TOC still works.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5028, T2683

Differential Revision: https://secure.phabricator.com/D9049