Merge pull request gitpython-developers#329 from cool-RR/patch-2

Be explicit about usage of commit of head-ref in index diff call.

There is no need to do this, but it won't hurt either.

Author: Byron

doc/source/tutorial.rst

@@ -393,7 +393,7 @@ Use the diff framework if you want to implement git-status like functionality.
 
 * A diff between the index and the commit's tree your HEAD points to
 
- * use ``repo.index.diff(repo.head)``
+ * use ``repo.index.diff(repo.head.commit)``
 
 * A diff between the index and the working tree