Correct Git repository browse behavior for differences in "ls-tree" output

Summary:
Ref T13589. The output for "git ls-tree commit:path" (the old invocation) and "git ls-tree commit -- path" (the new invocation) differs: the latter emits absolute paths.

Update the code to account for this difference in behavior.

Test Plan:

• Browsed a non-root directory in a Git repository in Diffusion.
• Before: saw absolute paths.
• After: saw relative paths.

Maniphest Tasks: T13589

Differential Revision: https://secure.phabricator.com/D21519