phabricator: migrate [phabricator.auth] to [auth]
This resurrects the warning mechanism removed in
20a4543e9a2b. I'm not worried
about the copy/paste of the code for a brief transitional period.
mercurial (__VERSION__) __CODENAME__; urgency=medium
* Automated build performed by upstream.
-- Mercurial Devel <mercurial-devel@mercurial-scm.org> __DATE__