#38 `debchange` in Ubuntu-derived systems uses non-default distribution name

package (1.2.3) bionic; urgency=medium 

package (1.2.3) unstable; urgency=medium 

On an Ubuntu-derived system the execution of `debchange` (as part of `make release-...`) results in the following header line: ``` package (1.2.3) bionic; urgency=medium ``` The expected header line everywhere else is: ``` package (1.2.3) unstable; urgency=medium ``` There is a [bug report](https://bugs.launchpad.net/ubuntu/+source/devscripts/+bug/1655717) about this behavior. We could work around this behavior by explicitly providing the `--distribution` argument with `unstable`. But this could potentially break other expectations. Thus the user should be able to disable this behavior.

fixed in e61890dc025fc71cc89ffeaf81dc146a0da220de