Fix changelog generation for anago usage

[saschagrunert]

• We now use the branch specified from anago to not assume a release branch in any case.
• The tag does not exist when generating the notes, so we have to use the HEAD as end rev and the latest tag as start rev.
• If we're not on a release branch, we also do not touch the CHANGELOG-x.y.md files of other releases
• When start or end sha is provided, we do not discover them any more

[k8s-ci-robot]

@saschagrunert: Adding label: do-not-merge/blocked-paths because PR changes a protected file.

Reasons for blocking this PR:

[Changes to certain release tools can affect our ability to test, build, and release Kubernetes. This PR must be explicitly approved by SIG Release repo admins.]

• anago

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[cpanato]

/lgtm

[justaugustus]

/lgtm
/approve

[saschagrunert]

/test pull-release-cluster-up

[hasheddan]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cpanato, hasheddan, justaugustus, saschagrunert

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• OWNERS [justaugustus]

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[justaugustus]

This one looks like it's going to get stuck on line 322 again...
/test pull-release-cluster-up