Bug in undo/redo handling in conjunction with history-beginning-search-backward

[Mikael Magnusson <mikachu@xxxxxxxxx>]

zsh -f
% bindkey -e
% bindkey '^U' undo
% bindkey '^Y' redo
% bindkey '^[C' history-beginning-search-backward
at this point, type in "bin" and invoke the
history-beginning-search-backward widget, type a space, then undo and
redo fully a few times, you'll notice the point where the extra "n"
starts popping into the command line.

I think Bart was the last one to poke the undo code, or maybe Oliver?
:) I haven't bisected it (I will if anyone asks me to), but it doesn't
happen in 5.0.7 and it does happen in zsh-5.1-21-ga4f087b (which is
just the other old binary i had lying around).

-- 
Mikael Magnusson