Fix test cases

Author: nvie

git/test/test_diff.py

@@ -21,7 +21,8 @@
     Repo,
     GitCommandError,
     Diff,
-    DiffIndex
+    DiffIndex,
+    NULL_TREE,
 )
 
 
@@ -132,13 +133,13 @@ def test_diff_initial_commit(self):
         initial_commit = self.rorepo.commit('33ebe7acec14b25c5f84f35a664803fcab2f7781')
 
         # Without creating a patch...
-        diff_index = initial_commit.diff('root')
+        diff_index = initial_commit.diff(NULL_TREE)
         assert diff_index[0].b_path == 'CHANGES'
         assert diff_index[0].new_file
         assert diff_index[0].diff == ''
 
         # ...and with creating a patch
-        diff_index = initial_commit.diff('root', create_patch=True)
+        diff_index = initial_commit.diff(NULL_TREE, create_patch=True)
         assert diff_index[0].b_path == 'CHANGES'
         assert diff_index[0].new_file
         assert diff_index[0].diff == fixture('diff_initial')
@@ -164,7 +165,7 @@ def test_diff_interface(self):
                 diff_item = commit.tree
             # END use tree every second item
 
-            for other in (None, 'root', commit.Index, commit.parents[0]):
+            for other in (None, NULL_TREE, commit.Index, commit.parents[0]):
                 for paths in (None, "CHANGES", ("CHANGES", "lib")):
                     for create_patch in range(2):
                         diff_index = diff_item.diff(other=other, paths=paths, create_patch=create_patch)