Fix diffusion browse queries in git

Summary:
If you try to load a directory in diffusion in a git repo that has a readme
file and you don't include a trailing slash in the path, you get an error
because we don't assemble the full paths of files correctly. This fixes that.

Test Plan: load such a directory in diffusion and see no fatal

Reviewers: epriestley, vrana

Reviewed By: vrana

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3533