pickaxe: die when --find-object and --pickaxe-all are combined

pickaxe: die when --find-object and --pickaxe-all are combined

Neither the --pickaxe-all documentation nor --find-object's has ever
suggested that you can combine the two. See f506b8e8b5 (git log/diff:
add -G<regexp> that greps in the patch text, 2010-08-23) and
15af58c1ad (diffcore: add a pickaxe option to find a specific blob,
2018-01-04).

But we've silently tolerated it, which makes the logic in
diffcore_pickaxe() harder to reason about. Let's assert that we won't
have the two combined.

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