repl: don't clobber RegExp.$ properties

In REPL, if we evaluate the `RegExp` object's predefined properties,
and if they happen to have the same expression, for example,

    > RegExp.$1
    'RegExp.$1'

then doing `eval(RegExp.$1)` would evaluate `RegExp.$1` recursively and
eventually throw `RangeError: Maximum call stack size exceeded`.

This patch stores the old values of `RegExp`'s predefined proprties in
an array and restores them just before the current expression entered
by user is evaluated.

Fixes: https://github.com/nodejs/io.js/issues/597
PR-URL: https://github.com/nodejs/io.js/pull/2137


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>