bug#12796: Optimize `ido-completing-read' for larger lists with flex matching enabled

[Stefan Monnier]

>> From this bit in ido-set-matches-1:

> (if ido-enable-prefix 
>     (setq re (concat "\\`" re)))

> It seems not including the leading and trailing .* is intended.

Indeed.  This undesired behavior was introduced by the change to
split-string introduced in Emacs-22, so the patch fixes a regression
w.r.t Emacs-21.

> So do you mind installing the following small change for 24.3 that
> greatly improves ido performance:

I guess it's OK, yes.


        Stefan