bug#38458: 27.0.50; case-insensitive substring completion

[Eli Zaretskii <eliz@gnu.org>]

> From: Stephen Berman <stephen.berman@gmx.net>
> Date: Mon, 02 Dec 2019 19:56:26 +0100
> 
> Typing TAB for case-insensitive substring completion of file and buffer
> names can put the cursor in the wrong position in the minibuffer.  To
> reproduce:
> 
> 0. $ mkdir /tmp/test; touch /tmp/test/{testing,Testing}
> 1. $ emacs-master -Q --eval "(setq read-buffer-completion-ignore-case t
>      read-file-name-completion-ignore-case t completion-category-overrides
>      '((buffer (styles substring)) (file (styles substring))))"
> 2. Type `C-x C-f /tmp/test/tes TAB'
> => The minibuffer displays this, with point (^) after `s' in `testing':
> Find file: /tmp/test/testing
>                         ^
> 3. Visit /tmp/test/testing and /tmp/test/Testing, then from a buffer
> other than these, e.g. /temp/test, type `C-x b tes TAB'
> 
> => The minibuffer displays this, with point (^) after `s' in `testing':
> Switch to buffer (default *scratch*): testing
>                                          ^
> 
> I think these are unintended side effects of these two changes:

If those changes caused this issue, then I think there was already a
bug in the substring completion style, which was just exposed by those
changes.

CC'ing Stefan in the hope that he could show us the light.