Prevent package publishing if not able to push changelog

There is a rare but possible condition when one can't push cangelog to github (e.g. github is unavailable). In this case debian/changelog of package may diverge from the one in repository.

It would be great If we have possibility to make this two actions one and "transactional". For instance, check possibility of push before duploading package.

The issue concerns the package publishing flow, the GitHub changelog push, and debian/changelog; start by tracing where changelog updates and duploading occur. Done means the package is not published when the changelog cannot be pushed, and the failure case is covered by a test.

Written by the indexing model from the issue text.