discovery: change users of `outgoing.missingheads` to `outgoing.ancestorsof` The attribute `missingheads` was recently renamed to `ancestorsof`, as it, despite the old name, doesn’t contain the missing heads but the changesets that were requested (including ancestors) for the outgoing operation. Changing all the users enables to print a warning if the old name is used. There is a good chance that some of the users are buggy because of the old name. Changing them to use the new name makes it more obvious that they are buggy. All users need to be reviewed for bugs. When sending patches for fixing them, the change will be more obvious without having to explain again and again the discrepancy of the old attribute name and what it actually contained.