RE: add non-chord keys to repeat isearch

[Drew Adams]

> Please don't.  These two keys are already used in Isearch Mode for
> scrolling.  To see this, set `isearch-allow-scroll' to t.  Then <next>
> and <prior> are handy keys for seeing more text around the 
> match without having to leave isearch and start again.

(I wasn't aware of that option. I've just filed a bug to index it in the Emacs
manual.)

However, just because some binding was made previously (when someone first had
the idea of scrolling without exiting Isearch) is no reason not to reconsider
that binding in light of a better suggestion. Emacs is not purely
first-come-first-served.

`M-v' and `C-v' can be used for scrolling (when the option is non-nil).
And `<prior>' and `<next>' can be used for repeat search. There's room for all.

[BTW, scrolling during isearch doesn't seem to work beyond a single screen
height. Is that a bug or a feature? IOW, `C-s C-v C-v': the second `C-v' has no
effect.]

> The same applies to any keys bound to commands which have the
> `isearch-scroll' non-nil.

That's fine, but a specific key binding can override that. We can choose to bind
`<prior>' to `isearch-repeat-backward' even though `scroll-up' has non-nil
property `isearch-scroll'. Works fine. There is no need to sacrifice all keys
that might be bound globally to `scroll-up'.