Speed up diffusion_browsequery for mercurial repositories

Summary:
Ref T4387. By using `hg locate` to attempt to only list files in the given path
browsing diffusion is a bit faster. In a repo of about 600M it shaves a rough 100ms
off viewing the root of the project.

Test Plan: Looked around in diffusion and saw it showed everything including .files, which was nice

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4387

Differential Revision: https://secure.phabricator.com/D8163