Use lexical-binding in bug-reference.el

* .dir-locals.el: Set bug-reference-url-format in all modes, not just
changelog mode. Use (eval . (bug-reference-mode)) as described
in (info "(emacs) Specifying File Variables")
* lisp/progmodes/bug-reference.el: Use lexical binding.
(bug-reference-unfontify):
(bug-reference-fontify): Mention args in docstring.

Bug#35123

.dir-locals.el

@@ -1,6 +1,7 @@
 ((nil . ((tab-width . 8)
          (sentence-end-double-space . t)
-         (fill-column . 70)))
+         (fill-column . 70)
+         (bug-reference-url-format . "https://debbugs.gnu.org/%s")))
  (c-mode . ((c-file-style . "GNU")
             (c-noise-macro-names . ("INLINE" "ATTRIBUTE_NO_SANITIZE_UNDEFINED" "UNINIT" "CALLBACK" "ALIGN_STACK"))
             (electric-quote-comment . nil)
@@ -12,8 +13,7 @@
                    (log-edit-setup-add-author . t)))
  (change-log-mode . ((add-log-time-zone-rule . t)
 		     (fill-column . 74)
-		     (bug-reference-url-format . "https://debbugs.gnu.org/%s")
-		     (mode . bug-reference)))
+		     (eval . (bug-reference-mode))))
  (diff-mode . ((mode . whitespace)))
  (emacs-lisp-mode . ((indent-tabs-mode . nil)
                      (electric-quote-comment . nil)

lisp/progmodes/bug-reference.el

@@ -1,4 +1,4 @@
-;; bug-reference.el --- buttonize bug references
+;; bug-reference.el --- buttonize bug references  -*- lexical-binding: t; -*-
 
 ;; Copyright (C) 2008-2019 Free Software Foundation, Inc.
 
@@ -91,15 +91,15 @@ The second subexpression should match the bug reference (usually a number)."
 (bug-reference-set-overlay-properties)
 
 (defun bug-reference-unfontify (start end)
-  "Remove bug reference overlays from region."
+  "Remove bug reference overlays from the region between START and END."
   (dolist (o (overlays-in start end))
     (when (eq (overlay-get o 'category) 'bug-reference)
       (delete-overlay o))))
 
 (defvar bug-reference-prog-mode)
 
 (defun bug-reference-fontify (start end)
-  "Apply bug reference overlays to region."
+  "Apply bug reference overlays to the region between START and END."
   (save-excursion
     (let ((beg-line (progn (goto-char start) (line-beginning-position)))
 	  (end-line (progn (goto-char end) (line-end-position))))