Don't expand empty [paths] so later interpolation can do the right thing.
Example:
[paths]
default =
default-push = %(default)s
# repo.py - repository base classes for mercurial
#
# Copyright 2005 Matt Mackall <mpm@selenic.com>
#
# This software may be used and distributed according to the terms
# of the GNU General Public License, incorporated herein by reference.
class RepoError(Exception): pass