builddeb: use the os codename instead of 'unstable' This fixes a lintian error (and indeed, launchpad rejects it) by using the distribution's codename (e.g. xenial, trusty, etc).

mercurial (__VERSION__) __CODENAME__; urgency=medium

  * Automated build performed by upstream.

 -- Mercurial Devel <mercurial-devel@selenic.com>  __DATE__