bug#16411: undo-only bugs

[Barry OReilly]

> undo-in-region does not implement undo-only, indeed. I think the way
> to implement undo-only is to change undo-make-selective-list so it
> also skips redo entries (depending on undo-no-redo, obviously) using
> the above while loop.

I don't see how that's a correct solution in the face of arbitrary
regional undos and prefix args. Would you have the redo records
resulting from regional undos still map to t in the undo-equiv-table?
How does your solution account for redo records that undid several
because of prefix-arg? undo-equiv-table only maps to the change group
after the last undone record without information about the (1-
prefix-arg) others.

My proposition accounts for these considerations and I think is
correct. If not I hope to find out when I start hacking it.