Run `git archive --remote=git@example.com:diffusion/ABC commit-ish path > path.tar` for example `git archive --remote=git@phabricator.local:diffusion/TEST HEAD testfile > testfile.tar`.
You can also just extract the file right away by piping to tar `git archive --remote=git@phabricator.local:diffusion/TEST HEAD testfile | tar -x`