Gerrit review tree viewer

Fetch open reviews from review.openstack.org and display them
as a plain text tree. Makes it easy to see which reviews depend
on other reviews, so you can focus on the head of a chain first.

usage: gtree.py [-h] [--project PROJECT] [--baseurl BASEURL] [--merged]

optional arguments:
  -h, --help         show this help message and exit
  --project PROJECT
  --baseurl BASEURL
  --merged           show merge counts broken down by owner