Fix regression when attempting to execute yarn version (yarnpkg#2491)

There was a conflict when commander attempts to parse the incoming args
between the command executed and the options since the name `version`
was shared. In other words, executing the command `yarn version`
would yield the same output as `yarn --version`. This relates to PR yarnpkg#2268.

src/cli/index.js

@@ -170,8 +170,13 @@ if (commandName === 'help' || args.indexOf('--help') >= 0 || args.indexOf('-h')
   process.exit(1);
 }
 
+if (!command) {
+  args.unshift(commandName);
+  command = commands.run;
+}
+invariant(command, 'missing command');
+
 // parse flags
-args.unshift(commandName);
 commander.parse(startArgs.concat(args));
 commander.args = commander.args.concat(endArgs);