notebook: fix, only one completion autopick

IPython/frontend/html/notebook/static/js/codecell.js

@@ -278,6 +278,7 b' var IPython = (function (IPython) {'
 
         // give common prefix of a array of string
         function sharedStart(A){
+            if(A.length == 1){return A[0]}
             if(A.length > 1 ){
                 var tem1, tem2, s, A = A.slice(0).sort();
                 tem1 = A[0];