RE: C-r and C-s in minibuffer should search completion

[Drew Adams]

> However, I think PgDn is a good candidate for a new command that
> just like PgUp displays a list of completions in a separate buffer,
> will display a list of elements from the history list in a
> separate buffer.  I think this would be a useful feature.
> It will allow browsing all elements of the history list in one buffer
> and using normal isearch to search them.

I proposed M-o, but which key you choose is not so important. 

However, I would argue to reserve the pair of keys `next' and `prior'
(sometimes labeled PgDn and PgUp) for something that, well, comes in pairs
and has some notion of, well, next and previous. That just makes sense.

There is no need to use a different buffer from *Completions*. When the user
hits M-o (or whatever), the current history list should simply become the
*Completions* set; that's all. Nothing special.

You don't need to use isearch to search the completion candidates (you can
complete against them, as usual), but you of course can (as always), just by
moving the cursor to buffer *Completions* and then using C-s.