Let diffusion.readmequery accept a commit

Summary:
Ref T2683. This should probably just be diffusion.filecontentquery but keep things as they are for now.

This method uses a commit, so accept one. Soon, this will save a bit of work.

Test Plan: Viewed readmes in main and browse views.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2683

Differential Revision: https://secure.phabricator.com/D9093