Fix diffusion.readmequery to work in a cluster enviroment

Summary:
Ref T2783. This method is kind of goofballs:

• We send a big list of paths to it.
• It sends back a giant blob of HTML.

Instead, just figure out the path we want locally, then fetch the content with diffusion.filecontentquery.

Test Plan:

• Viewed main view and directory view, saw a README.
• See screenshots.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2783

Differential Revision: https://secure.phabricator.com/D11099