repl: make tab completion read up the prototype of "global"

For example, there's a global "toString()" function, so the REPL's
tab completion should pick that up.

lib/repl.js

@@ -535,6 +535,10 @@ REPLServer.prototype.complete = function(line, callback) {
         if (this.useGlobal ||
             this.context.constructor &&
             this.context.constructor.name === 'Context') {
+          var contextProto = this.context;
+          while (contextProto = Object.getPrototypeOf(contextProto)) {
+            completionGroups.push(Object.getOwnPropertyNames(contextProto));
+          }
           completionGroups.push(Object.getOwnPropertyNames(this.context));
           addStandardGlobals();
           completionGroupsLoaded();

test/simple/test-repl-tab-complete.js

@@ -202,3 +202,9 @@ testMe.complete(' ', function(error, data) {
   assert.deepEqual(data, [[],undefined]);
   clearTimeout(spaceTimeout);
 });
+
+// tab completion should pick up the global "toString" object, and
+// any other properties up the "global" object's prototype chain
+testMe.complete('toSt', function(error, data) {
+  assert.deepEqual(data, [['toString'], 'toSt']);
+});