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 Plan: Unit tests. @asherkin, maybe you can confirm this? I can't actually get the warning, but I think my `hg` in PATH is just a bit out of date.

Reviewers: asherkin, btrahan

Reviewed By: asherkin

CC: asherkin, aran

Maniphest Tasks: T615, T4237

Differential Revision: https://secure.phabricator.com/D7784