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).

contrib/builddeb

@@ -69,6 +69,7 b' if [ "$BUILD" ]; then'
 
     sed -i.tmp "s/__VERSION__/$debver/" $changelog
     sed -i.tmp "s/__DATE__/$(date --rfc-2822)/" $changelog
+    sed -i.tmp "s/__CODENAME__/$CODENAME/" $changelog
     rm $changelog.tmp
 
     debuild -us -uc -b

contrib/debian/changelog

@@ -1,4 +1,4 b''
-mercurial (__VERSION__) unstable; urgency=medium
+mercurial (__VERSION__) __CODENAME__; urgency=medium
 
   * Automated build performed by upstream.