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>

diff.h

@@ -326,6 +326,8 @@ extern void diff_setup_done(struct diff_options *);
 #define DIFF_PICKAXE_KIND_S	4 /* traditional plumbing counter */
 #define DIFF_PICKAXE_KIND_G	8 /* grep in the patch */
 
+#define DIFF_PICKAXE_KINDS_MASK (DIFF_PICKAXE_KIND_S | DIFF_PICKAXE_KIND_G)
+
 #define DIFF_PICKAXE_IGNORE_CASE	32
 
 extern void diffcore_std(struct diff_options *);