repl: don’t complete non-simple expressions

Change the regular expression that recognizes “simple” JS expressions
to requiring that the full line needs to match it.

Previously, in terms like `a().b.`, `b.` would be a partial match.
This meant that completion would evaluate `b` and either fail with
a `ReferenceError` or, if `b` was some global, return the properties
of the global `b` object.

PR-URL: https://github.com/nodejs/node/pull/6192


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