release_tool: Fix problems when no previous version of a repo exists.
release_tool: Fix problems when no previous version of a repo exists.

There was an exception and the changelog did not get a correct entry.

Changelog: None

Signed-off-by: Kristian Amlie <505e66ae45028a0596c853559221f0b72c1cee21@northern.tech>

pasinskim/integration,pasinskim/integration,pasinskim/integration