avoid calling (cd ...) with `nil' as a directory name
When (hg-root) cannot determine the current Mercurial root, print an
error message instead of trying to evaluate (cd nil).
to build a new macosx binary package:
install macpython from http://www.python.org/download/mac
install py2app from http://pythonmac.org/packages/
make sure /usr/local/bin is in your path
run bdist_mpkg in top-level hg directory
find packaged stuff in dist directory