merge: replace a repo.lookup('.') by more typical repo['.'].node()
The `repo.lookup('.')` form comes from b3311e26f94f (merge: fix
--preview to show all nodes that will be merged (issue2043).,
2010-02-15). I don't know why that commit changed from `repo['.']`,
but I don't think there's any reason to do that. Note that performance
should not be a reason (anymore?), because repo.lookup() is
implemented by first creating a context object.
Differential Revision: https://phab.mercurial-scm.org/D7998
cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=python "$RUNTESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF