Fix `arc diff --no-ansi`

Summary: Currently, we get an exception on empty %Ls for `arc diff --no-ansi` or similar (see P698).

Test Plan: Ran `arc diff --no-ansi`.

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4624