packaging: add Bullseye, remove Jessie stable

packaging: add Bullseye, remove Jessie

Jessie is oldoldstable now, and Bullseye is going to be the next stable (now
testing). We're continuing to support the current oldstable, stable and testing
releases.

Differential Revision: https://phab.mercurial-scm.org/D6762

--- a/contrib/packaging/Makefile	Sun Aug 25 19:38:09 2019 +0700
+++ b/contrib/packaging/Makefile	Sun Aug 25 19:46:24 2019 +0700
@@ -1,9 +1,9 @@
 $(eval HGROOT := $(shell cd ../..; pwd))
 
 DEBIAN_CODENAMES := \
-  jessie \
   stretch \
-  buster
+  buster \
+  bullseye
 
 UBUNTU_CODENAMES := \
   xenial \