Fix hg push and hg push -r sometimes creating new heads without --force. Fixing issue179. The algorithm checks if there not more new heads on the remote side than heads which become non-heads due to getting children. Pushing this repo: m /\ 3 3a| |/ / 2 2a |/ 1 to a repo only having 1, 2 and 3 didn't abort requiring --force before. Added test cases for this and some doc strings for used methods.

# 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