git-submodule: provide easy way of adding new submodules

git-submodule: provide easy way of adding new submodules

To make a submodule effectively usable, the path and
a URL where the submodule can be cloned need to be stored
in .gitmodules.  This subcommand takes care of setting
this information after cloning the new submodule.
Only the index is updated, so, if needed, the user may still
change the URL or switch to a different branch of the submodule
before committing.

Signed-off-by: Sven Verdoolaege <redacted>
Signed-off-by: Junio C Hamano <redacted>