Fixed previously failing test

It actually revealed a bug in the implementation of Submodule.add,
which just showed in python 3 for the wrong reasons. Thankfully,
failing tests after all allowed to get this issue fixed ... .

Author: Byron

git/objects/submodule/base.py

@@ -256,7 +256,7 @@ def add(cls, repo, name, path, url=None, branch=None, no_checkout=False):
         # END assure url correctness
 
         # INSTANTIATE INTERMEDIATE SM
-        sm = cls(repo, cls.NULL_BIN_SHA, cls.k_default_mode, path, name)
+        sm = cls(repo, cls.NULL_BIN_SHA, cls.k_default_mode, path, name, url='invalid-temporary')
         if sm.exists():
             # reretrieve submodule from tree
             try:
@@ -298,7 +298,7 @@ def add(cls, repo, name, path, url=None, branch=None, no_checkout=False):
             if not branch_is_default:
                 kwargs['b'] = br.name
             # END setup checkout-branch
-            mrepo = git.Repo.clone_from(url, path, **kwargs)
+            mrepo = git.Repo.clone_from(url, os.path.join(repo.working_tree_dir, path), **kwargs)
         # END verify url
 
         # update configuration and index

git/test/test_submodule.py

@@ -610,6 +610,7 @@ def test_add_empty_repo(self, rwdir):
         git.Repo.init(empty_repo_dir)
 
         for checkout_mode in range(2):
-            self.failUnlessRaises(ValueError, parent.create_submodule, 'empty', 'empty',
-                                  url=empty_repo_dir, no_checkout=checkout_mode)
+            name = 'empty' + str(checkout_mode)
+            self.failUnlessRaises(ValueError, parent.create_submodule, name, name,
+                                  url=empty_repo_dir, no_checkout=checkout_mode and True or False)
         # end for each checkout mode