git submodule: Add test cases for git submodule add

git submodule: Add test cases for git submodule add

Add simple test cases for adding and initialising submodules. The
init step is necessary in order to verify the added information.

The second test exposes a known breakage due to './' in the path: git
ls-files simplifies the path but git add does not, which leads to git
init looking for different lines in .gitmodules than git add adds.

The other tests add test cases for '//' and '..' in the path which
currently fail for the same reason.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>