Improve reproducibility of generated loaddefs file.

* lisp/emacs-lisp/autoload.el (autoload-generate-file-autoloads):
Make the return value the modtime of the input file (if no autoloads).
(update-directory-autoloads): In the "no autoloads" section,
use "most recent modtime" rather than "current time".

; http://lists.gnu.org/archive/html/emacs-devel/2015-06/msg00688.html

lisp/emacs-lisp/autoload.el

@@ -522,8 +522,8 @@ If OUTFILE is non-nil and FILE specifies a `generated-autoload-file'
 different from OUTFILE, then OUTBUF is ignored.
 
 Return non-nil if and only if FILE adds no autoloads to OUTFILE
-\(or OUTBUF if OUTFILE is nil)."
+\(or OUTBUF if OUTFILE is nil).  The actual return value is
-  (catch 'done
+FILE's modification time."
   (let (load-name
 	(print-length nil)
 	(print-level nil)
@@ -534,6 +534,8 @@ Return non-nil if and only if FILE adds no autoloads to OUTFILE
 	(absfile (expand-file-name file))
 	;; nil until we found a cookie.
 	output-start)
+    (when
+	(catch 'done
 	  (with-current-buffer (or visited
 				   ;; It is faster to avoid visiting the file.
 				   (autoload-find-file file))
@@ -631,7 +633,8 @@ Return non-nil if and only if FILE adds no autoloads to OUTFILE
 	  (or (not output-start)
 	      ;; If the entries were added to some other buffer, then the file
 	      ;; doesn't add entries to OUTFILE.
-          otherbuf))))
+	      otherbuf))
+      (nth 5 (file-attributes absfile)))))
 
 (defun autoload-save-buffers ()
   (while autoload-modified-buffers
@@ -757,7 +760,7 @@ write its autoloads into the specified file instead."
 						  t files-re))
 			       dirs)))
          (done ())
-	 (this-time (current-time))
+	 (last-time)
          ;; Files with no autoload cookies or whose autoloads go to other
          ;; files because of file-local autoload-generated-file settings.
 	 (no-autoloads nil)
@@ -782,14 +785,14 @@ write its autoloads into the specified file instead."
 		   ;; There shouldn't be more than one such entry.
 		   ;; Remove the obsolete section.
 		   (autoload-remove-section (match-beginning 0))
-		   (let ((last-time (nth 4 form)))
+		   (setq last-time (nth 4 form))
 		   (dolist (file file)
 		     (let ((file-time (nth 5 (file-attributes file))))
 		       (when (and file-time
 				  (not (time-less-p last-time file-time)))
 			 ;; file unchanged
 			 (push file no-autoloads)
-			   (setq files (delete file files)))))))
+			 (setq files (delete file files))))))
 		  ((not (stringp file)))
 		  ((or (not (file-exists-p file))
                        ;; Remove duplicates as well, just in case.
@@ -811,14 +814,18 @@ write its autoloads into the specified file instead."
             (push file done)
 	    (setq files (delete file files)))))
       ;; Elements remaining in FILES have no existing autoload sections yet.
+      (let ((no-autoloads-time (or last-time '(0 0 0 0))) file-time)
 	(dolist (file files)
 	  (cond
 	   ((member (expand-file-name file) autoload-excludes) nil)
 	   ;; Passing nil as second argument forces
 	   ;; autoload-generate-file-autoloads to look for the right
 	   ;; spot where to insert each autoloads section.
-         ((autoload-generate-file-autoloads file nil buffer-file-name)
+	   ((setq file-time
-          (push file no-autoloads))))
+		  (autoload-generate-file-autoloads file nil buffer-file-name))
+	    (push file no-autoloads)
+	    (if (time-less-p no-autoloads-time file-time)
+		(setq no-autoloads-time file-time)))))
 
 	(when no-autoloads
 	  ;; Sort them for better readability.
@@ -827,8 +834,8 @@ write its autoloads into the specified file instead."
 	  (goto-char (point-max))
 	  (search-backward "\f" nil t)
 	  (autoload-insert-section-header
-	 (current-buffer) nil nil no-autoloads this-time)
+	   (current-buffer) nil nil no-autoloads no-autoloads-time)
-	(insert generate-autoload-section-trailer))
+	  (insert generate-autoload-section-trailer)))
 
       (let ((version-control 'never))
 	(save-buffer))