Enable git diff-tree with empty second tree-ish param

AUTHORS

@@ -26,5 +26,6 @@ Contributors are:
 -Mikuláš Poul <mikulaspoul _at_ gmail.com>
 -Charles Bouchard-Légaré <cblegare.atl _at_ ntis.ca>
 -Yaroslav Halchenko <debian _at_ onerussian.com>
+-Stefan Gangefors
 
 Portions derived from other open source works and are clearly marked.

git/diff.py

@@ -81,7 +81,8 @@ def diff(self, other=Index, paths=None, create_patch=False, **kwargs):
 
         :param other:
             Is the item to compare us with.
-            If None, we will be compared to the working tree.
+            If None, it will be compared to the working tree.
+            If empty string, it will be compared to it's parent with diff-tree.
             If Treeish, it will be compared against the respective tree
             If Index ( type ), it will be compared against the index.
             If git.NULL_TREE, it will compare against the empty tree.
@@ -132,7 +133,8 @@ def diff(self, other=Index, paths=None, create_patch=False, **kwargs):
             diff_cmd = self.repo.git.diff_tree
         elif other is not None:
             args.insert(0, '-r')  # recursive diff-tree
-            args.insert(0, other)
+            if other != '':
+                args.insert(0, other)
             diff_cmd = self.repo.git.diff_tree
 
         args.insert(0, self)

git/test/test_diff.py

@@ -266,3 +266,17 @@ def test_diff_interface(self):
         cp = c.parents[0]
         diff_index = c.diff(cp, ["does/not/exist"])
         self.assertEqual(len(diff_index), 0)
+
+    def test_diff_tree_empty_other(self):
+        initial_commit = self.rorepo.commit('9066712dca21ef35c534e068f2cc4fefdccf1ea3')
+
+        # Test that we can use an empty string as other in a diff.
+        # It should do a diff-tree without the second tree-ish param
+        diff_index = initial_commit.diff('')
+        self.assertEqual(len(diff_index), 16)
+        self.assertEqual(diff_index[0].change_type, 'M')
+        self.assertTrue(diff_index[12].deleted_file)
+        self.assertTrue(diff_index[14].renamed)
+        self.assertEqual(diff_index[14].rename_from, 'test/asserts.py')
+        self.assertEqual(diff_index[14].rename_to, 'test/testlib/asserts.py')
+        self.assertEqual(diff_index[15].change_type, 'A')