repl: default useGlobal to true

This is a partial revert of
15157c3c. This change lead to a
regression that broke require() in the CLI REPL, as imported
files were evaluated in a different context.

Refs: https://github.com/nodejs/node/pull/5703
Fixes: https://github.com/nodejs/node/issues/7788
PR-URL: https://github.com/nodejs/node/pull/7795


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>