diff --git a/lib/grit/repo.rb b/lib/grit/repo.rb
index 44567d61..0218f96e 100644
--- a/lib/grit/repo.rb
+++ b/lib/grit/repo.rb
@@ -564,7 +564,7 @@ def log(commit = 'master', path = nil, options = {})
     #   +b+ is the other commit
     #   +paths+ is an optional list of file paths on which to restrict the diff
     def diff(a, b, *paths)
-      diff = self.git.native('diff', {}, a, b, '--', *paths)
+      diff = self.git.native('diff', {:full_index => true}, a, b, '--', *paths)
 
       if diff =~ /diff --git a/
         diff = diff.sub(/.*?(diff --git a)/m, '\1')