git submodule: Remove now obsolete tests before cloning a repo

git submodule: Remove now obsolete tests before cloning a repo

Since 55892d23 "git clone" itself checks that the destination path is not
a file but an empty directory if it exists, so there is no need anymore
for module_clone() to check that too.

Two tests have been added to test the behavior of "git submodule add" when
path is a file or a directory (A subshell had to be added to the former
last test to stay in the right directory).

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>