Allow ignoring local variable values permanently

* doc/lispref/variables.texi (File Local Variables): Document it.

* lisp/files.el (ignored-local-variable-values): New user option
(bug#5003).
(hack-local-variables-confirm): Allow ignoring permanently.
(hack-local-variables-filter): Ignore the permanently ignored
variables.

doc/lispref/variables.texi

@@ -1995,6 +1995,16 @@ Doing so adds those variable/value pairs to
 file.
 @end defopt
 
+@defopt ignored-local-variable-values
+If there are some local variables that you always want to always
+ignore, this variable can be used.  It uses the same syntax as
+@code{safe-local-variable-values}, but the variable/value pairs here
+will always be ignored when handling local variables.  As with that
+variable, when Emacs queries the user about whether to obey a
+file-local variable, the user can choose to ignore them permanently,
+and that will alter this variable and save it to the user's custom file.
+@end defopt
+
 @defun safe-local-variable-p sym val
 This function returns non-@code{nil} if it is safe to give @var{sym}
 the value @var{val}, based on the above criteria.

etc/NEWS

@@ -2222,6 +2222,12 @@ This command, called interactively, toggles the local value of
 
 ** Miscellaneous
 
++++
+*** New user option 'ignored-local-variable-values'.
+This is the opposite of 'safe-local-variable-values' -- it's an alist
+of local variables (and accompanying values) that are to be ignored
+when reading a local variable section from a file.
+
 ---
 *** 'indent-tabs-mode' is now a global minor mode instead of just a variable.
 

lisp/files.el

@@ -3437,7 +3437,19 @@ in order to initialize other data structure based on them.")
 (defcustom safe-local-variable-values nil
   "List variable-value pairs that are considered safe.
 Each element is a cons cell (VAR . VAL), where VAR is a variable
-symbol and VAL is a value that is considered safe."
+symbol and VAL is a value that is considered safe.
+
+Also see `ignored-local-variable-values'."
+  :risky t
+  :group 'find-file
+  :type 'alist)
+
+(defcustom ignored-local-variable-values nil
+  "List variable-value pairs that will be ignored.
+Each element is a cons cell (VAR . VAL), where VAR is a variable
+symbol and VAL is a value that will be ignored.
+
+Also see `safe-local-variable-values'."
   :risky t
   :group 'find-file
   :type 'alist)
@@ -3592,7 +3604,9 @@ n  -- to ignore the local variables list.")
 	(if offer-save
 	    (insert "
 !  -- to apply the local variables list, and permanently mark these
-      values (*) as safe (in the future, they will be set automatically.)\n\n")
+      values (*) as safe (in the future, they will be set automatically.)
+i  -- to ignore the local variables list, and permanently mark these
+      values (*) as ignored\n\n")
 	  (insert "\n\n"))
 	(dolist (elt all-vars)
 	  (cond ((member elt unsafe-vars)
@@ -3616,16 +3630,24 @@ n  -- to ignore the local variables list.")
 	(pop-to-buffer buf '(display-buffer--maybe-at-bottom))
 	(let* ((exit-chars '(?y ?n ?\s))
 	       (prompt (format "Please type %s%s: "
-			       (if offer-save "y, n, or !" "y or n")
+			       (if offer-save "y, n, ! or i" "y or n")
 			       (if (< (line-number-at-pos (point-max))
 				      (window-body-height))
 				   ""
 				 ", or C-v/M-v to scroll")))
 	       char)
-	  (if offer-save (push ?! exit-chars))
+	  (when offer-save
+            (push ?i exit-chars)
+            (push ?! exit-chars))
 	  (setq char (read-char-choice prompt exit-chars))
-	  (when (and offer-save (= char ?!) unsafe-vars)
-	    (customize-push-and-save 'safe-local-variable-values unsafe-vars))
+	  (when (and offer-save
+                     (or (= char ?!) (= char ?i))
+                     unsafe-vars)
+	    (customize-push-and-save
+             (if (= char ?!)
+                 'safe-local-variable-values
+               'ignored-local-variable-values)
+             unsafe-vars))
 	  (prog1 (memq char '(?! ?\s ?y))
 	    (quit-window t)))))))
 
@@ -3718,13 +3740,18 @@ If these settings come from directory-local variables, then
 DIR-NAME is the name of the associated directory.  Otherwise it is nil."
   ;; Find those variables that we may want to save to
   ;; `safe-local-variable-values'.
-  (let (all-vars risky-vars unsafe-vars)
+  (let (all-vars risky-vars unsafe-vars ignored)
     (dolist (elt variables)
       (let ((var (car elt))
 	    (val (cdr elt)))
 	(cond ((memq var ignored-local-variables)
 	       ;; Ignore any variable in `ignored-local-variables'.
 	       nil)
+              ((seq-some (lambda (elem)
+                           (and (eq (car elem) var)
+                                (eq (cdr elem) val)))
+                         ignored-local-variable-values)
+               nil)
 	      ;; Obey `enable-local-eval'.
 	      ((eq var 'eval)
 	       (when enable-local-eval