Correct Diffusion browse behavior when visiting a path URI with no trailing slash

Summary:
See PHI1983. Ref T13599. Ref T13589. Currently, if you browse to a path browse URI in Diffusion without a trailing slash (/browse/master/src), you get a nonsensical view (the directory as a single item).

Be more precise in how "git ls-tree" arguments are constructed.

Test Plan: Visited files and directories in the browse view, with and without trailing slashes. Saw improved behavior for directories with no trailing slash and reasonable behavior in all other cases.

Maniphest Tasks: T13599, T13589

Differential Revision: https://secure.phabricator.com/D21528