diff: introduce DIFF_PICKAXE_KINDS_MASK

diff: introduce DIFF_PICKAXE_KINDS_MASK

Currently the check whether to perform pickaxing is done via checking
`diffopt->pickaxe`, which contains the command line argument that we
want to pickaxe for. Soon we'll introduce a new type of pickaxing, that
will not store anything in the `.pickaxe` field, so let's migrate the
check to be dependent on pickaxe_opts.

It is not enough to just replace the check for pickaxe by pickaxe_opts,
because flags might be set, but pickaxing was not requested ('-i').
To cope with that, introduce a mask to check only for the bits indicating
the modes of operation.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>