R1:b0f7e7a6af2d

Work around `hg` echoing warnings to stdout under --debug

Summary:
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.

Test…