repl: fix tab completion for a non-global context

Use vm.isContext() to properly identify contexts.

PR-URL: https://github.com/joyent/node/pull/25382
PR-URL: https://github.com/nodejs/io.js/pull/2052


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>