cocci: refactor common patterns to use xstrdup_or_null()

d64ea0f83b ("git-compat-util: add xstrdup_or_null helper",
2015-01-12) added a handy wrapper that allows us to get a duplicate
of a string or NULL if the original is NULL, but a handful of
codepath predate its introduction or just weren't aware of it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -791,8 +791,7 @@ struct ref_update *ref_transaction_add_update(
 		hashcpy(update->new_sha1, new_sha1);
 	if (flags & REF_HAVE_OLD)
 		hashcpy(update->old_sha1, old_sha1);
-	if (msg)
-		update->msg = xstrdup(msg);
+	update->msg = xstrdup_or_null(msg);
 	return update;
 }