Make repo branch for coverage diff configurable
Make repo branch for coverage diff configurable

We can use some custom branch during code checkout, so let's
provide an ability for CI to redefine the branch

Metaswitch/calico,neiljerram/felix,Metaswitch/calico,matthewdupre/felix,matthewdupre/felix,neiljerram/felix,neiljerram/felix,neiljerram/felix