codereview: re-enable 15-second status prints

Also print all status changes when using hg -v.

The start_status_thread call was lost during the refactoring in
https://codereview.appspot.com/5395044/diff/6006/lib/codereview/codereview.py
Oops.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7834044