Do not do anything if there's no new commit

Right now, when there's no change, we still add "Start using
obs-service-github_tarballs" .changes entries and rename the tarball
(because the version is different -- we use current time for version).
This is just wrong. If there's no new commit, then clearly, there's
nothing new.
Merge pull request #3 from vuntz/master

Do not do anything if there's no new commit