tests/test-issue433

protocol: unify changegroup commands

- add sendchangegroup protocol helpers
- handle commands with None results
- move changegroup commands into wireproto.py

#!/bin/sh
# http://mercurial.selenic.com/bts/issue433

hg init a
cd a
echo a > a
hg commit -Ama

hg parents -r 0 doesnotexist
true