Add --raw output format and (some) options to invoke rename/copy
detection on the diff.

This will probably prevent many lookup/free
operations in calling code.

When the examples/diff.c was written, there was not yet a revparse
API.  Now we can use it to make command line parsing way better
with less code.  Yay!

The version field is overwritten otherwise. The opt instance is
already initialized properly.

The diff constructor functions had some confusing names, where the
"old" side of the diff was coming after the "new" side.  This
reverses the order in the function name to make it less confusing.

Specifically...

* git_diff_index_to_tree becomes git_diff_tree_to_index
* git_diff_workdir_to_index becomes git_diff_index_to_workdir
* git_diff_workdir_to_tree becomes git_diff_tree_to_workdir

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.