bug#29360: 26.0; Add full-buffer choice for `isearch-lazy-highlight'

[Juri Linkov]

>> Are you sure this should be part of isearch, not hi-lock?
>
> No relation to hi-lock. This is not about font-lock
> highlighting.

hi-lock is not about font-lock highlighting too
when font-lock is not active.  See the variable
‘hi-lock-highlight-range’ with its default limit of 200000.

I'm thinking about using lazy-highlight in hi-lock
to overcome this limitation after adding support
for full-buffer to lazy-highlight.  Maybe better
to generalize the current implementation of
isearch-lazy-highlight-update to apply a function
given as an argument to perform different actions
on the found matches (by default, adding the lazy-highlight
overlay, and for hi-lock adding the hi-lock-overlay).