Support "root" as a special value in .diff() calls

This enabled getting diff patches for root commits.

git/diff.py

@@ -49,6 +49,7 @@ def diff(self, other=Index, paths=None, create_patch=False, **kwargs):
             If None, we will be compared to the working tree.
             If Treeish, it will be compared against the respective tree
             If Index ( type ), it will be compared against the index.
+            If the string 'root', it will compare the empty tree against this tree.
             It defaults to Index to assure the method will not by-default fail
             on bare repositories.
 
@@ -87,10 +88,15 @@ def diff(self, other=Index, paths=None, create_patch=False, **kwargs):
         if paths is not None and not isinstance(paths, (tuple, list)):
             paths = [paths]
 
-        if other is not None and other is not self.Index:
-            args.insert(0, other)
+        diff_cmd = self.repo.git.diff
         if other is self.Index:
-            args.insert(0, "--cached")
+            args.insert(0, '--cached')
+        elif other == 'root':
+            args.insert(0, '--root')
+            diff_cmd = self.repo.git.diff_tree
+        elif other is not None:
+            args.insert(0, other)
+            diff_cmd = self.repo.git.diff_tree
 
         args.insert(0, self)
 
@@ -101,7 +107,7 @@ def diff(self, other=Index, paths=None, create_patch=False, **kwargs):
         # END paths handling
 
         kwargs['as_process'] = True
-        proc = self.repo.git.diff(*self._process_diff_args(args), **kwargs)
+        proc = diff_cmd(*self._process_diff_args(args), **kwargs)
 
         diff_method = Diff._index_from_raw_format
         if create_patch:

git/test/fixtures/diff_initial

@@ -0,0 +1,10 @@
+--- /dev/null
++++ b/CHANGES
+@@ -0,0 +1,7 @@
++=======
++CHANGES
++=======
++
++0.1.0
++=====
++initial release

git/test/test_diff.py

@@ -128,6 +128,21 @@ def test_diff_index_raw_format(self):
         assert res[0].deleted_file
         assert res[0].b_path == ''
 
+    def test_diff_initial_commit(self):
+        initial_commit = self.rorepo.commit('33ebe7acec14b25c5f84f35a664803fcab2f7781')
+
+        # Without creating a patch...
+        diff_index = initial_commit.diff('root')
+        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)
+        assert diff_index[0].b_path == 'CHANGES'
+        assert diff_index[0].new_file
+        assert diff_index[0].diff == fixture('diff_initial')
+
     def test_diff_patch_format(self):
         # test all of the 'old' format diffs for completness - it should at least
         # be able to deal with it
@@ -149,7 +164,7 @@ def test_diff_interface(self):
                 diff_item = commit.tree
             # END use tree every second item
 
-            for other in (None, commit.Index, commit.parents[0]):
+            for other in (None, 'root', 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)