ob-core: add binding for merged-params to avoid multiple evaluation

* lisp/ob-core.el (org-babel-execute-src-block): Add binding for
  merged-params to avoid multiple evaluation of
  `org-babel-merge-params´.

lisp/ob-core.el

@@ -521,15 +521,13 @@ Optionally supply a value for PARAMS which will be merged with
 the header arguments specified at the front of the source code
 block."
   (interactive)
-  (let ((info (or info (org-babel-get-src-block-info))))
+  (let* ((info (or info (org-babel-get-src-block-info)))
+	 (merged-params (org-babel-merge-params (nth 2 info) params)))
     (when (org-babel-confirm-evaluate
-	   (let ((i info))
-	     (setf (nth 2 i) (org-babel-merge-params (nth 2 info) params))
-	     i))
+	   (let ((i info)) (setf (nth 2 i) merged-params) i))
       (let* ((lang (nth 0 info))
 	     (params (if params
-			 (org-babel-process-params
-			  (org-babel-merge-params (nth 2 info) params))
+			 (org-babel-process-params merged-params)
 		       (nth 2 info)))
 	     (cache-p (and (not arg) (cdr (assoc :cache params))
 			  (string= "yes" (cdr (assoc :cache params)))))