Fix changelog task to handle github's "Merged PR..."

This was a real peace of work. I have a hunch that there is a better way (please point me to it, if you know) but the current approach is to inject tokens to know what goes to where. If there is the infamous Merged pull request something comment we consider the first line up until the "\n" to be the real comment. Everything else gets deleted.

Fixes #912 

Let me know what you think :-)