R1:d6fd36570407

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…