repl: use String#repeat instead of Array#join

String#repeat is quite a bit faster than new Array().join().

PR-URL: https://github.com/nodejs/node/pull/3900


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>