submodule: port submodule subcommand 'foreach' from shell to C

submodule: port submodule subcommand 'foreach' from shell to C

This aims to make git-submodule foreach a builtin. 'foreach' is ported to
the submodule--helper, and submodule--helper is called from
git-submodule.sh.

Helped-by: Brandon Williams <bmwill@google.com>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Prathamesh Chavan <pc44800@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>