Adjust thumbs to new rename-file behavior

* etc/NEWS: Mention this.
* lisp/thumbs.el (thumbs-rename-images): Treat the destination
as special only if it is a directory name.  When there is
a marked list, turn the destination into a directory name
if it is not already.

etc/NEWS

@@ -1287,7 +1287,7 @@ can be written as (rename-file C (file-name-as-directory D)), a
 formulation portable to both older and newer versions of Emacs.
 Affected functions include add-name-to-file, copy-directory,
 copy-file, format-write-file, gnus-copy-file, make-symbolic-link,
-rename-file, and write-file.
+rename-file, thumbs-rename-images, and write-file.
 
 
 * Lisp Changes in Emacs 26.1

lisp/thumbs.el

@@ -523,23 +523,16 @@ Open another window."
   (interactive "FRename to file or directory: ")
   (let ((files (or thumbs-marked-list (list (thumbs-current-image))))
 	failures)
-    (if (and (not (file-directory-p newfile))
-	     thumbs-marked-list)
-	(if (file-exists-p newfile)
-	    (error "Renaming marked files to file name `%s'" newfile)
-	  (make-directory newfile t)))
+    (when thumbs-marked-list
+      (make-directory newfile t)
+      (setq newfile (file-name-as-directory newfile)))
     (if (yes-or-no-p (format "Really rename %d files? " (length files)))
 	(let ((thumbs-file-list (thumbs-file-alist))
 	      (inhibit-read-only t))
 	  (dolist (file files)
 	    (let (failure)
 	      (condition-case ()
-		  (if (file-directory-p newfile)
-		      (rename-file file
-				   (expand-file-name
-				    (file-name-nondirectory file)
-				    newfile))
-		    (rename-file file newfile))
+		  (rename-file file newfile)
 		(file-error (setq failure t)
 			    (push file failures)))
 	      (unless failure