diff: migrate diff_flags.pickaxe_ignore_case to a pickaxe_opts bit

Currently flags for pickaxing are found in different places. Unify the
flags into the `pickaxe_opts` field, which will contain any pickaxe related
flags.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/diff.h b/diff.h
index ea310f7..8af1213 100644
--- a/diff.h
+++ b/diff.h
@@ -91,7 +91,6 @@ struct diff_flags {
 	unsigned override_submodule_config:1;
 	unsigned dirstat_by_line:1;
 	unsigned funccontext:1;
-	unsigned pickaxe_ignore_case:1;
 	unsigned default_follow_renames:1;
 };
 
@@ -327,6 +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_IGNORE_CASE	32
+
 extern void diffcore_std(struct diff_options *);
 extern void diffcore_fix_diff_index(struct diff_options *);