submodule add: fix breakage when re-adding a deep submodule

submodule add: fix breakage when re-adding a deep submodule

Since recently a submodule with name <name> has its git directory in the
.git/modules/<name> directory of the superproject while the work tree
contains a gitfile pointing there.

When the same submodule is added on a branch where it wasn't present so
far (it is not found in the .gitmodules file), the name is not initialized
from the path as it should. This leads to a wrong path entered in the
gitfile when the .git/modules/<name> directory is found, as this happily
uses the - now empty - name. It then always points only a single directory
up, even if we have a path deeper in the directory hierarchy.

Fix that by initializing the name of the submodule early in module_clone()
if module_name() returned an empty name and add a test to catch that bug.

Reported-by: Jehan Bing <jehan@orb.com>
Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>