pickaxe: refactor function selection in diffcore-pickaxe()

pickaxe: refactor function selection in diffcore-pickaxe()

It's hard to read this codepath at a glance and reason about exactly
what combination of -G and -S will compile either regexes or kwset,
and whether we'll then dispatch to "diff_grep" or "has_changes".

Then in the "--find-object" case we aren't using the callback
function, but were previously passing down "has_changes".

Refactor this code to exhaustively check "opts", it's now more obvious
what callback function (or none) we want under what mode.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>