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>

Author: jlehmann

git-submodule.sh

@@ -131,6 +131,7 @@ module_clone()
 	gitdir=
 	gitdir_base=
 	name=$(module_name "$path" 2>/dev/null)
+	test -n "$name" || name="$path"
 	base_path=$(dirname "$path")
 
 	gitdir=$(git rev-parse --git-dir)

t/t7406-submodule-update.sh

@@ -611,4 +611,12 @@ test_expect_success 'submodule update places git-dir in superprojects git-dir re
 	)
 '
 
+test_expect_success 'submodule add properly re-creates deeper level submodules' '
+	(cd super &&
+	 git reset --hard master &&
+	 rm -rf deeper/ &&
+	 git submodule add ../submodule deeper/submodule
+	)
+'
+
 test_done