Clamp negative rev numbers at zero.
Prior to this change, trying to run "hg log -r -50:" in a repo with
less than 50 changes caused an error. Now that we clamp at zero: no
more error.
# 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