Ref T615. Ref T4237. With --debug, Mercurial will echo an "ignoring untrusted configuration option" warning to stdout if .hgrc has the wrong owner.
However, we need --debug to make {parents} usable, at least until the patches I got into the upstream are widely deployed. So after getting --debug output, strip off any leading warnings.
These warnings should always be in English, at least, since we set LANG explicitly.