Is undo-tree dependency strictly necessary

[Leo Alekseyev]

I was wondering if undo-tree is only there to match Vim behavior, and
if I could use Evil without it.  I actually find built-in Emacs undo
functionality intuitive and I'm used to how it operates.  On the other
hand, I've run into some cases where undo-tree didn't behave as
expected.  If there are no ill consequences in using original Emacs
undo vs undo-tree, it would be nice if it were configurable.

--Leo

P.S.: Thanks Frank and Vegard for quick bug-fixing action!