builtin/notes: convert to struct object_id

Convert most of the static functions to use struct object_id.  In
addition, convert copy_notes_for_rewrite and its callers.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/commit.c b/builtin/commit.c
index da1ba4c..7587810 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1809,7 +1809,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		cfg = init_copy_notes_for_rewrite("amend");
 		if (cfg) {
 			/* we are amending, so current_head is not NULL */
-			copy_note_for_rewrite(cfg, current_head->object.oid.hash, oid.hash);
+			copy_note_for_rewrite(cfg, &current_head->object.oid, &oid);
 			finish_copy_notes_for_rewrite(cfg, "Notes added by 'git commit --amend'");
 		}
 		run_rewrite_hook(&current_head->object.oid, &oid);