bug#33838: 27.0.50; lazy-highlight fails with a quick key sequence

[Juri Linkov]

> Quickly typing `C-s C-s' to repeat the last search in a file with long
> wrapped lines sometimes fails to lazy-highlight other matches.
>
> This is because the condition in isearch-lazy-highlight-new-loop relies on
> the return value of `(sit-for 0)' and when there is input pending like the
> second `C-s' keystroke unprocessed due to the large file size with long
> lines, it doesn't start a new lazy-highlight loop.

I realized that sit-for here is for a reason to not try running
isearch-lazy-highlight-new-loop on every keypress.  The problem is that
when the first `C-s' is ignored (in this case isearch-string is an empty
string and isearch-lazy-highlight-last-string is the last search string),
for the second `C-s', isearch-string is equal to 
isearch-lazy-highlight-last-string
from the last search, so a new loop is not initiated.

A proper fix would be to reset isearch-lazy-highlight-last-string
on exiting Isearch, so it won't affect the next search:

diff --git a/lisp/isearch.el b/lisp/isearch.el
index dd0973d4ea..d2c1c150b5 100644
--- a/lisp/isearch.el
+++ b/lisp/isearch.el
@@ -1357,6 +1357,7 @@ isearch-done
   (setq minibuffer-message-timeout isearch-original-minibuffer-message-timeout)
   (isearch-dehighlight)
   (lazy-highlight-cleanup lazy-highlight-cleanup)
+  (setq isearch-lazy-highlight-last-string nil)
   (let ((found-start (window-group-start))
        (found-point (point)))
     (when isearch-window-configuration