New hook, lazy-count-update-hook

New hook, lazy-count-update-hook

* lisp/isearch.el (lazy-count-update-hook): New hook allowing to
display the lazy count in special ways.
(isearch-lazy-highlight-new-loop,
isearch-lazy-highlight-buffer-update): Run `lazy-count-update-hook' at
appropriate times.
* lisp/comint.el (comint-history-isearch-setup,
comint-history-isearch-end): Make sure no lazy count is displayed.
* lisp/simple.el (minibuffer-history-isearch-setup): Make sure no lazy
count is displayed.