Fix an issue with browsing repositories that have README, etc

Summary: D3533 changed the path for files at root from, e.g., "README" to "/README", which fatals when we try to git cat-file it.

Test Plan:
This no longer happens:

{F24874}

Viewed a directory in browse without a trailing slash in the URL.

Reviewers: vrana, nh

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4016