node: replace nullid and friends with nodeconstants class
The introduction of 256bit hashes require changes to nullid and other
constant magic values. Start pushing them down from repository and
revlog where sensible.
Differential Revision: https://phab.mercurial-scm.org/D9465
cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=$PYTHON "$RUNTESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF