Fix changelog generation for custom forks
Fix changelog generation for custom forks

We now default to the base k/k repository when doing `krel stage` with a
custom Kubernetes fork. This allows us to generate the changelog from a
single source of truth, because we cannot know which modifications have
been done to the fork or if the tags/release branches are available
there.

Signed-off-by: Sascha Grunert <70ab469ddb2ac3e35f32ed7c2fd1cca514b2e879@redhat.com>

kubernetes/release,kubernetes/release