undo misbehaves when data structures are involved

[Spacechild1]

Steps to reproduce:

1. open the help patch of [append]
2. create a [+] object anywhere in the patch
3. click on the [traverse pd-help-append-data, bang( message
4. enter a number into the number box on the left to create a new scalar
5. undo -> the [+] disappears and there is now a second number box conntected to the [trigger] object

If you skip step 2, step 5 will still add the bogus number box (although it shouldn't do anything at all).