bug#17453: Isearch doesn't work properly with Follow Mode.

[Juri Linkov]

> If Follow Mode is not enabled, all the above functions do the Right
> Thing on the single window.

>From another perspective, settings lazy-highlight-buffer to t
(implemented in bug#21092) and removing the current restriction of
(overlay-put ov 'window (selected-window)) will lazy-highlight matches
in all follow windows with no effort.