diff: convert diff_flush_patch_id to struct object_id

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/diff.h b/diff.h
index fcf334b..150f1c5 100644
--- a/diff.h
+++ b/diff.h
@@ -354,7 +354,7 @@ extern int run_diff_files(struct rev_info *revs, unsigned int option);
 extern int run_diff_index(struct rev_info *revs, int cached);
 
 extern int do_diff_cache(const struct object_id *, struct diff_options *);
-extern int diff_flush_patch_id(struct diff_options *, unsigned char *, int);
+extern int diff_flush_patch_id(struct diff_options *, struct object_id *, int);
 
 extern int diff_result_code(struct diff_options *, int);