clone: pass --single-branch during --recurse-submodules

clone: pass --single-branch during --recurse-submodules

Previously, performing "git clone --recurse-submodules --single-branch"
resulted in submodules cloning all branches even though the superproject
cloned only one branch. Pipe --single-branch through the submodule
helper framework to make it to 'clone' later on.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>