c47ef57 (diff: introduce diff.submodule configuration variable,
2012-11-13) introduced the diff.submodule configuration variable, but
forgot to teach git-completion.bash about it. Fix this.
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>