Lazy highlighting now happens immediately when the search string is at
least as long as the value of the new custom variable
`lazy-highlight-no-delay-length`. Also avoid updating the lazy
count in the echo area too often.
* isearch.el (lazy-highlight-no-delay-length): New defcustom.
* isearch.el (lazy-lazy-count-format): Avoid a momentarily incorrect
count when reversing search direction.
* isearch.el (isearch-lazy-highlight-new-loop): Avoid a call to
`isearch-message` that is quickly succeed by a second echo area
update, thus causing flicker.
* isearch.el (isearch-lazy-highlight-new-loop):
Start lazy highlight immediately if appropriate.
* etc/NEWS: Announce the change.
* doc/emacs/search.texi: Document `lazy-highlight-no-delay-length'.
Copyright-paperwork-exempt: yes
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