Add stub for commit, extra check.

dulwich/index.py

@@ -181,3 +181,11 @@ class Index(object):
     def update(self, entries):
         for name, value in entries.iteritems():
             self[name] = value
+
+
+def commit(index, basepath, object_store):
+    """Commit an index.
+
+    :return: SHA1 of the created tree.
+    """
+    raise NotImplementedError(commit)

dulwich/tests/test_objects.py

@@ -63,6 +63,7 @@ class BlobReadTests(unittest.TestCase):
         sha = 'e69de29bb2d1d6434b8b29ae775ad8c2e48c5391'
         b = self.get_blob(sha)
         self.assertEqual(b.data, '')
+        self.assertEqual(b.id, sha)
         self.assertEqual(b.sha().hexdigest(), sha)
   
     def test_create_blob_from_string(self):