REPL fixes

This PR contains three commits:

• A port of https://github.com/joyent/node/issues/8874, which has already landed on joyent/node.
• A port of https://github.com/joyent/node/pull/25382, which has not yet landed.
• A new commit, which fixes test-repl-tab-complete.js, which was silently not working as expected. The test was relying on assertions that were never executed.