Use timestamp based packagename always with -t

Previously, package type (unstable, testing, stable)
were decided based on the git tag's status and -t
was only available in unstable commit.
With this change, we always use timestamp based
formatting when -t is given.

Change-Id: I6af5cd5f31574e578091d2ec1a67e246a361ef4d
Add git short sha1 to timestamp

Change-Id: I74ecc8928193e1d55b8e4130daed70bf5488fa79
Merge changes from topic 'pkg2-juno' into stable/juno

* changes:
  Add git short sha1 to timestamp
  Use timestamp based packagename always with -t