bug#26274: 26.0.50; completion regression

[Stephen Berman]

Substring completion involving non-ascii characters (with or without
case-folding) does not work in current master as it had till recently
and still does in Emacs 25.2 (my most recent build from master is from
March 15 and it also shows the problem).  To reproduce:

0. In a shell: cd /tmp; touch atest Atest ätest Ätest testä testÄ
1. Let ~/.emacs consist of the following:
--8<---------------cut here---------------start------------->8---
(custom-set-variables
 ;; custom-set-variables was added by Custom.
 ;; If you edit it by hand, you could mess it up, so be careful.
 ;; Your init file should contain only one such instance.
 ;; If there is more than one, they won't work right.
 '(completion-category-overrides
   (quote
    ((buffer
      (styles substring))
     (file
      (styles substring)))))
 '(read-buffer-completion-ignore-case t)
 '(read-file-name-completion-ignore-case t))
(custom-set-faces
 ;; custom-set-faces was added by Custom.
 ;; If you edit it by hand, you could mess it up, so be careful.
 ;; Your init file should contain only one such instance.
 ;; If there is more than one, they won't work right.
)
--8<---------------cut here---------------end--------------->8---
2. Start emacs; after typing `C-x C-f test TAB' the *Completions* buffer
   shows all six files from step 0. 
3. With both `C-x C-f atest TAB' and `C-x C-f atest TAB' the
   *Completions* buffer shows both atest and Atest. 
4. With `C-x C-f ätest TAB', `C-x C-f Ätest TAB', `C-x C-f testä TAB',
   or `C-x C-f testÄ TAB', the minibuffer displays "[Sole completion]".
   In 25.2 with the first two, the *Completions* buffer shows both ätest
   and Ätest and with the last two, both testä and testÄ.


In GNU Emacs 26.0.50 (build 4, x86_64-pc-linux-gnu, GTK+ Version 3.20.10)
 of 2017-03-27 built on rosalinde
Repository revision: bd074d9bab470da24dd79491d8c25b22c2a5bb3d
Windowing system distributor 'The X.Org Foundation', version 11.0.11803000
System Description:     openSUSE Leap 42.2

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