avoid duplicates in history

[Dan Nicolaescu <dann@ics.uci.edu>]

One feature of tcsh that I miss in emacs is "histdup=erase". When
histdup is set that way when a command is entered in history all
previous occurrences of the same command will be erased. 
bash3 will also implement this feature, it will be called
"erasedups". 

Here is an implementation of this feature for Emacs. 

If this is acceptable, I will provide a complete patch with
documentation. 

Feedback would be appreciated. 
Is the history-erasedups name ok? 

Thanks.


*** minibuf.c	27 May 2004 23:22:35 -0700	1.266
--- minibuf.c	01 Jun 2004 09:47:22 -0700	
***************
*** 61,66 ****
--- 61,71 ----
  
  Lisp_Object Qhistory_length, Vhistory_length;
  
+ /* No duplicates in history.  */
+ 
+ int history_erasedups;
+ 
+ 
  /* Fread_minibuffer leaves the input here as a string. */
  
  Lisp_Object last_minibuf_string;
***************
*** 742,748 ****
  	{
  	  Lisp_Object length;
  
! 	  histval = Fcons (histstring, histval);
  	  Fset (Vminibuffer_history_variable, histval);
  
  	  /* Truncate if requested.  */
--- 747,754 ----
  	{
  	  Lisp_Object length;
  
! 	  if (history_erasedups) Fdelete (histstring, histval);
!           histval = Fcons (histstring, histval);
  	  Fset (Vminibuffer_history_variable, histval);
  
  	  /* Truncate if requested.  */
***************
*** 2631,2636 ****
--- 2637,2647 ----
  property of a history variable overrides this default.  */);
    XSETFASTINT (Vhistory_length, 30);
  
+   DEFVAR_BOOL ("history-erasedups", &history_erasedups,
+ 	       doc: /* *Non-nil means delete duplicates in history.
+   */);
+   history_erasedups = 0;
+   
    DEFVAR_LISP ("completion-auto-help", &Vcompletion_auto_help,
  	       doc: /* *Non-nil means automatically provide help for invalid completion input.  */);
    Vcompletion_auto_help = Qt;