Diffusion - replace last `hg manifest` call with `hg locate`

Summary: Fixes T4387.

Test Plan: Setup a mercurial repository for rabbitmq-server. Browsed around it and things looked good.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4387

Differential Revision: https://secure.phabricator.com/D10380