git submodule foreach: Provide access to submodule name, as '$name'

git submodule foreach: Provide access to submodule name, as '$name'

The argument to 'git submodule foreach' already has access to the variables
'$path' (the path to the submodule, relative to the superproject) and '$sha1'
(the submodule commit recorded by the superproject).

This patch adds another variable -- '$name' -- which contains the name of the
submodule, as recorded in the superproject's .gitmodules file.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>