clone: allow an explicit argument for parallel submodule clones Just pass it along to "git submodule update", which may pick reasonable defaults if you don't specify an explicit number. Reviewed-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Stefan Beller <sbeller@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>