This makes the diff functions that take callbacks both take
the payload parameter after the callback function pointers and
pass the payload as the last argument to the callback function
instead of the first. This should make them consistent with
other callbacks across the API.
There are a lot of places where the diff API gives the user access
to internal data structures and many of these were being exposed
through non-const pointers. This replaces them all with const
pointers for any object that the user can access but is still
owned internally to the git_diff_list or git_diff_patch objects.
This will probably break some bindings... Sorry!
