bug#16411: undo-only bugs

[Barry OReilly]

> I've never heard anyone bump into this "Y undo-redos of X" problem,
> so I don't think optimizing it is worth the trouble.

Happens to me all the time.

> If anything should be done with it, I think it'd be to *cut* the
> extra undo/redo pairs.

No complaint from me. That would change the behavior of ordinary undo
command, which you just said you don't want to change.

> I'm not completely convinced that this generator is worthwhile

Ok, I'll lose it then.

>> I originally set out to do this, but making the weak references
>> work seemed overly tricky to me. The value stored in
>> undo-redo-table would need to be non weak with weak references to
>> undo elements. I supposed this would mean many one element weak
>> hash tables. That seems dodgy.

> Hmm... that's a very good point. Worth mentioning in a comment.

You actually want me to do that? That is: wrap every referenced
element in a size 1 weak hash table. Are you sure that gives the
memory savings over the element level mapping of the undo-redo-table
in the patch?