prepare_changelog: Update regex to work with GNU grep
prepare_changelog: Update regex to work with GNU grep

grubernaut/packer,grubernaut/packer,ricardclau/packer,timsutton/packer,hashicorp/packer,dave2/packer,rnaveiras/packer,mitchellh/packer,pecigonzalo/packer,ricardclau/packer,timsutton/packer,ricardclau/packer,ricardclau/packer,paulmey/packer,dave2/packer,paulmey/packer,dave2/packer,ricardclau/packer,pecigonzalo/packer,rnaveiras/packer,grubernaut/packer,grubernaut/packer,hashicorp/packer,dave2/packer,hashicorp/packer,timsutton/packer,dave2/packer,ricardclau/packer,paulmey/packer,timsutton/packer,rnaveiras/packer,mitchellh/packer,mitchellh/packer,paulmey/packer,pecigonzalo/packer,grubernaut/packer,mitchellh/packer,hashicorp/packer,rnaveiras/packer,pecigonzalo/packer,mitchellh/packer,timsutton/packer,grubernaut/packer,paulmey/packer,timsutton/packer,rnaveiras/packer,rnaveiras/packer,dave2/packer,paulmey/packer