bug#9918: 24.0.90; lazy-highlighting in Info mode

[Juri Linkov <juri@jurta.org>]

> From "emacs -Q":
> 1. Open Info node "(emacs)Top" (C-h r).
> 2. Move point to the end of the first paragraph.
> 3. Type "C-s extensible".
>
> The word "extensible" (in the first line of the paragraph) should be
> lazy-highlighted, as happens if you repeat the experiment after doing
> "M-x text-mode".

The difference between this recipe in a normal buffer and an Info buffer
is in the line

    (when (not isearch-error)

in `isearch-lazy-highlight-new-loop'.  A normal Isearch starts
a new lazy highlighting loop even if there is no success
(`isearch-success' is nil) and no errors (`isearch-error' is nil).

But Info Isearch sets `isearch-error' to "initial node", so
`isearch-lazy-highlight-new-loop' doesn't start lazy-highlighting.
It seems with the error "initial node", it's still possible to
lazy-highlight other matches.  So it's not a normal error,
and we can make an exception for this type of "error".
This will work with the following patch:

=== modified file 'lisp/isearch.el'
--- lisp/isearch.el	2011-10-30 14:41:20 +0000
+++ lisp/isearch.el	2011-10-31 23:12:01 +0000
@@ -2721,7 +2721,7 @@ (defun isearch-lazy-highlight-new-loop (
     ;; something important did indeed change
     (lazy-highlight-cleanup t) ;kill old loop & remove overlays
     (setq isearch-lazy-highlight-error isearch-error)
-    (when (not isearch-error)
+    (when (or (not isearch-error) (equal isearch-error "initial node"))
       (setq isearch-lazy-highlight-start-limit beg
 	    isearch-lazy-highlight-end-limit end)
       (setq isearch-lazy-highlight-window       (selected-window)

But it's not good to hardcode it, so perhaps we should introduce
a special property for errors that are not quite an error,
and set it just for "initial node".