diff: properly error out when combining multiple pickaxe options

diff: properly error out when combining multiple pickaxe options

In f506b8e8b5 (git log/diff: add -G<regexp> that greps in the patch text,
2010-08-23) we were hesitant to check if the user requests both -S and
-G at the same time. Now that the pickaxe family also offers --find-object,
which looks slightly more different than the former two, let's add a check
that those are not used at the same time.

Signed-off-by: Stefan Beller <redacted>
Signed-off-by: Junio C Hamano <redacted>