Fixes to "hg heads -r FOO":
Make it actually work (undefined variable 'rev'; allow to pass a rev parameter).
repo.branchlookup() doesn't need a copy of heads because it doesn't modify it.
Use None as default argument to heads() instead of nullid.
Doc string PEPification.
# 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