meteor shell should preserve global _

2443d83 (in 1.0.4) ensures that the repl doesn't overwrite `_`, but it
replaces the server's global `_` with shell-server's `_`.  This changes
appears to preserve it.

Fixes meteor#4010.

tools/server/shell-server.js

@@ -159,9 +159,14 @@ Sp.startREPL = function startREPL(options) {
   // History persists across shell sessions!
   self.initializeHistory();
 
+  // Save the global `_` object in the server.  This is probably defined by the
+  // underscore package.  It is unlikely to be the same object as the `var _ =
+  // require('underscore')` in this file!
+  var originalUnderscore = repl.context._;
+
   Object.defineProperty(repl.context, "_", {
     // Force the global _ variable to remain bound to underscore.
-    get: function () { return _; },
+    get: function () { return originalUnderscore; },
 
     // Expose the last REPL result as __ instead of _.
     set: function(lastResult) {