more efficient org-babel-with-temp-filebuffer

* lisp/ob-tangle.el (org-babel-with-temp-filebuffer): Don't execute
  macro argument multiple times.

lisp/ob-tangle.el

@@ -122,13 +122,15 @@ represented in the file."
 `progn', then kill the FILE buffer returning the result of
 evaluating BODY."
   (declare (indent 1))
-  (let ((temp-result (make-symbol "temp-result"))
+  (let ((temp-path (make-symbol "temp-path"))
+	(temp-result (make-symbol "temp-result"))
 	(temp-file (make-symbol "temp-file"))
 	(visited-p (make-symbol "visited-p")))
-    `(let (,temp-result ,temp-file
-           (,visited-p (get-file-buffer ,file)))
-       (org-babel-find-file-noselect-refresh ,file)
-       (setf ,temp-file (get-file-buffer ,file))
+    `(let* ((,temp-path ,file)
+	    (,visited-p (get-file-buffer ,temp-path))
+	    ,temp-result ,temp-file)
+       (org-babel-find-file-noselect-refresh ,temp-path)
+       (setf ,temp-file (get-file-buffer ,temp-path))
        (with-current-buffer ,temp-file
 	 (setf ,temp-result (progn ,@body)))
        (unless ,visited-p (kill-buffer ,temp-file))