Fix another Git 2.16.0 CLI compatibility issue

Summary:
This command also needs a "." instead of an empty string now.

(This powers the file browser typeahead in Diffusion.)

Test Plan: Will test in production since there's still no easy 2.16 installer for macOS.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19010