range-diff: move the diffopt initialization down one layer

range-diff: move the diffopt initialization down one layer

It is actually only the `output()` function that uses those diffopts. By
moving the diffopt initialization down into that function, it is
encapsulated better.

Incidentally, it will also make it easier to implement the `--left-only`
and `--right-only` options in `git range-diff` because the `output()`
function is now receiving all range-diff options as a parameter, not
just the diffopts.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>