repl: allow overriding builtins

Don't give names of built-in libraries special treatment.
Changes the REPL's behavior from this:

  > var path = 42
  > path
  A different "path" already exists globally

To this:

  > var path = 42
  > path
  42

Fixes #4512.

lib/repl.js

@@ -220,15 +220,10 @@ function REPLServer(prompt, stream, eval_, useGlobal, ignoreUndefined) {
 
     // Check if a builtin module name was used and then include it
     // if there's no conflict.
-    if (exports._builtinLibs.indexOf(cmd) !== -1) {
+    if (!(cmd in self.context) && exports._builtinLibs.indexOf(cmd) !== -1) {
       var lib = require(cmd);
-      if (cmd in self.context && lib !== self.context[cmd]) {
-        self.outputStream.write('A different "' + cmd +
-                                '" already exists globally\n');
-      } else {
       self.context._ = self.context[cmd] = lib;
       self.outputStream.write(self.writer(lib) + '\n');
-      }
       self.displayPrompt();
       return;
     }

test/simple/test-repl-autolibs.js

@@ -67,7 +67,7 @@ function test2(){
     gotWrite = true;
     if (data.length) {
       // repl response error message
-      assert.equal(data.indexOf('A different'), 0);
+      assert.equal(data, '{}\n');
       // original value wasn't overwritten
       assert.equal(val, global.url);
     }