docs: def _get_rev_num(self, rev=None):
  		"""
  		Determine the revision number for a given revision specifier.
  		"""
  		# first, determine the numeric ID
  		cmd = ['identify', '--num']
  		# workaround for #4
  		cmd.extend(['--config', 'defaults.identify='])
  		if rev:
  			cmd.extend(['--rev', rev])
  		res = self._invoke(*cmd)
  		return res.strip()