Feature: make function to ff branches to a certain 

According to our posters, and way of working:
https://www.praqma.com/stories/git-phlow/

We want to have the branches "master", "stable", "release" to indicate where in the release train we are.

This should be a function in PiP like:

`promotebranchToHere('stable')`

It should fail if it is not a FF merge, as per blogpost.