Fix latest tag retrieval for krel changelog

[saschagrunert]

What type of PR is this?

/kind bug

What this PR does / why we need it:

The issue fixed with this commit is that we cannot say if the target tag
to be built already exists in the local repository, which made it
impossible to krel to behave in an expected way. If a tag during
changelog generation already locally exists (but not remotely) seems to
differ in anago, which is the reason we now use a different approach:

1. We correctly chose the release branch (or master) HEAD from the
   remote as end SHA because we cannot know which branch is currently
   checked out.
2. We use a new helper function git.LatestGitHubTagsPerBranch which
   fetches the latest GitHub tags for each release branch and the
   master. This follows the logic that alpha releases are only on the
   master branch, whereas all other releases except the final tag are
   available only on release-x.y branches. The latest tag for the
   release branch (or master) will be our start commit for the release
   notes generation.

Which issue(s) this PR fixes:

Refers to kubernetes/sig-release#999

Special notes for your reviewer:

The original relnotes script used this implementation to achieve the same goal:

release/lib/gitlib.sh

Lines 163 to 186 in cadfbd5

	gitlib::last_releases () {
	local release
	local branch_name
	local latest_branch
	declare -Ag LAST_RELEASE
	logecho -n "Setting last releases by branch: "
	for release in $($GHCURL $K8S_GITHUB_API/releases|\
	jq -r '.[] | select(.draft==false) | .tag_name'); do
	# Alpha releases only on master branch
	if [[ $release =~ -alpha ]]; then
	LAST_RELEASE[master]=${LAST_RELEASE[master]:-$release}
	elif [[ $release =~ v([0-9]+\.[0-9]+)\.([0-9]+(-.+)?) ]]; then
	# Latest vx.x.0 release goes on both master and release branch.
	if [[ ${BASH_REMATCH[2]} == "0" ]]; then
	LAST_RELEASE[master]=${LAST_RELEASE[master]:-$release}
	fi
	branch_name=release-${BASH_REMATCH[1]}
	LAST_RELEASE[$branch_name]=${LAST_RELEASE[$branch_name]:-$release}
	fi
	done
	logecho -r "$OK"
	}

The general implementation has the drawback that we cannot generate changelogs for old releases any more, because we now rely on the latest tag from GitHub.

Does this PR introduce a user-facing change?:

- Updated `krel changelog` to correctly choose the start tag for a release

[cpanato]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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• cmd/krel/OWNERS [saschagrunert]
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