Simplify execution from "big if statement"

This code is functionally equivalent, but in a simpler form.  Now new
parameters to `execFile` do not require `exec` to be refactored.

lib/child_process.js

@@ -12,11 +12,9 @@ var spawn = exports.spawn = function (path, args /*, options OR env, customFds *
 };
 
 exports.exec = function (command /*, options, callback */) {
-  if (arguments.length < 3) {
-    return exports.execFile("/bin/sh", ["-c", command], arguments[1]);
-  } else {
-    return exports.execFile("/bin/sh", ["-c", command], arguments[1], arguments[2]);
-  }
+  var _slice = Array.prototype.slice;
+  var args = ["/bin/sh", ["-c", command]].concat(_slice.call(arguments, 1));
+  return exports.execFile.apply(this, args)
 };