hg push ends with an exception

[Impact] pushing to repositories does not work

[Development Fix] a fix was released with version 1.3 in precise
[Stable Fix] backport of the 1.3 fix

[Test Case]
create svn repo with standard layout (trunk/branches/tags)
put this in ~/.hgrc
[extensions]
hgsubversion =
hg clone svn+ssh://url/to/repo
cd repo
edit something
hg add, hg commit
hg push

expected: successful push
real result: traceback (see original report)

[Regression Potential]
low the change is small and simple

original report:
Mercurial repository created from SVN with hg clone.

Ubuntu 11.10
Mercurial 1.9.1-1ubuntu0.1
hgsubversion 1.2.1-2.1

Seems like this bugs: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=646946 and https://bitbucket.org/durin42/hgsubversion/issue/307/hg-push-to-svn-raises-exception

> hg push
pushing to svn+https://...
** unknown exception encountered, please report by visiting
** http://mercurial.selenic.com/wiki/BugTracker
** Python 2.7.2+ (default, Oct 4 2011, 20:03:08) [GCC 4.6.1]
** Mercurial Distributed SCM (version 1.9.1)
** Extensions loaded: extdiff, hgsubversion, rebase
Traceback (most recent call last):
  File "/usr/bin/hg", line 38, in <module>
    mercurial.dispatch.run()
  File "/usr/lib/python2.7/dist-packages/mercurial/dispatch.py", line 27, in run
    sys.exit(dispatch(request(sys.argv[1:])))
  File "/usr/lib/python2.7/dist-packages/mercurial/dispatch.py", line 64, in dispatch
    return _runcatch(req)
  File "/usr/lib/python2.7/dist-packages/mercurial/dispatch.py", line 87, in _runcatch
    return _dispatch(req)
  File "/usr/lib/python2.7/dist-packages/mercurial/dispatch.py", line 679, in _dispatch
    cmdpats, cmdoptions)
  File "/usr/lib/python2.7/dist-packages/mercurial/dispatch.py", line 454, in runcommand
    ret = _runcommand(ui, options, cmd, d)
  File "/usr/lib/python2.7/dist-packages/mercurial/dispatch.py", line 733, in _runcommand
    return checkargs()
  File "/usr/lib/python2.7/dist-packages/mercurial/dispatch.py", line 687, in checkargs
    return cmdfunc()
  File "/usr/lib/python2.7/dist-packages/mercurial/dispatch.py", line 676, in <lambda>
    d = lambda: util.checksignature(func)(ui, *args, **cmdoptions)
  File "/usr/lib/python2.7/dist-packages/mercurial/util.py", line 385, in check
    return func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/mercurial/extensions.py", line 137, in wrap
    util.checksignature(origfn), *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/mercurial/util.py", line 385, in check
    return func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/hgext/hgsubversion/wrappers.py", line 510, in generic
    return orig(ui, repo, *args, **opts)
  File "/usr/lib/python2.7/dist-packages/mercurial/util.py", line 385, in check
    return func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/mercurial/commands.py", line 3884, in push
    newbranch=opts.get('new_branch'))
  File "/usr/lib/python2.7/dist-packages/hgext/hgsubversion/svnrepo.py", line 48, in wrapper
    return fn(self, *args, **opts)
  File "/usr/lib/python2.7/dist-packages/hgext/hgsubversion/svnrepo.py", line 59, in push
    return wrappers.push(self, remote, force, revs)
  File "/usr/lib/python2.7/dist-packages/hgext/hgsubversion/wrappers.py", line 134, in push
    cmdutil.bail_if_changed(repo)
AttributeError: 'module' object has no attribute 'bail_if_changed'