Reduce flicker in Isearch mode

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

1 files changed, 7 insertions, 0 deletions

diff --git a/doc/emacs/search.texi b/doc/emacs/search.texi
index 637867b8115..f3c42bcea7f 100644
--- a/doc/emacs/search.texi
+++ b/doc/emacs/search.texi
@@ -2027,6 +2027,13 @@ highlighting:
 @item lazy-highlight-initial-delay
 @vindex lazy-highlight-initial-delay
 Time in seconds to wait before highlighting visible matches.
+Applies only if the search string is less than
+@code{lazy-highlight-no-delay-length} characters long.
+
+@item lazy-highlight-no-delay-length
+@vindex lazy-highlight-no-delay-length
+For search strings at least as long as the value of this variable,
+lazy highlighting of matches starts immediately.
 
 @item lazy-highlight-interval
 @vindex lazy-highlight-interval