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

This enabled getting diff patches for root commits.

1 files changed, 10 insertions, 4 deletions

diff --git a/git/diff.py b/git/diff.py
index 062220df..eada73b9 100644
--- a/git/diff.py
+++ b/git/diff.py
@@ -49,6 +49,7 @@ class Diffable(object):
             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 @@ class Diffable(object):
         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 @@ class Diffable(object):
         # 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: