* message.el (message-expand-group): Pass the common

prefix substring of completion to `display-completion-list'.

* mh-comp.el (mh-complete-word): Pass the common
prefix substring of completion to `display-completion-list'.

* dabbrev.el (dabbrev-completion): Pass the common
prefix substring of completion to `display-completion-list'.

* filecache.el (file-cache-minibuffer-complete)
(file-cache-complete): Ditto.

* tempo.el (tempo-display-completions): Ditto.

* wid-edit.el (widget-file-complete, widget-color-complete): Ditto.

* emacs-lisp/lisp.el (lisp-complete-symbol): Ditto.

* eshell/em-hist.el (eshell-list-history): Ditto.

* mail/mailabbrev.el (mail-abbrev-complete-alias): Ditto.

* progmodes/etags.el (complete-tag): Ditto.

* progmodes/make-mode.el (makefile-complete): Ditto.

* progmodes/meta-mode.el (meta-complete-symbol): Ditto.

* progmodes/octave-mod.el (octave-complete-symbol): Ditto.

* progmodes/pascal.el (pascal-complete-word)
(pascal-show-completions): Ditto.

* textmodes/bibtex.el (bibtex-complete-internal): Ditto.

* simple.el (completion-common-substring): New variable.
(completion-setup-function): Use `completion-common-substring'
to put faces.

* minibuf.c (Fdisplay_completion_list): Add new optional
argument COMMON_SUBSTRING. Bind `completion-common-substring'
to the optional argument during running `completion-setup-hook'.

em-hist.el

@@ -507,7 +507,7 @@ See also `eshell-read-history'."
 	;; Change "completion" to "history reference"
 	;; to make the display accurate.
 	(with-output-to-temp-buffer history-buffer
-	  (display-completion-list history)
+	  (display-completion-list history prefix)
 	  (set-buffer history-buffer)
 	  (forward-line 3)
 	  (while (search-backward "completion" nil 'move)