Create a diff with the difference between two tree objects.
This is equivalent to git diff  <old -tree>  <new -tree>
The first tree will be used for the "old_file" side of the delta and the
second tree will be used for the "new_file" side of the delta.  You can
pass NULL to indicate an empty tree, although it is an error to pass
NULL for both the old_tree and new_tree.