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.

The diff API is not in the parameter order one would expect from
other libgit2 APIs.  This fixes that.

This adds an API to generate a complete single-file patch text
from a git_diff_patch object.

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!

This should restore the ability to include libgit2 headers
in C++ projects.

Cherry picked 2de60205 from
development into new-error-handling.

Update the callback to provide some information related to the file change being processed and the range of the hunk, when applicable.