Fix invalid previous tag on krel changelog for beta releases > 0

The changelog for v1.19.0-beta.1 is diffing against the wrong version
(the latest alpha):
https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-1.19.md/#changelog-since-v1190-alpha3

This is a regression introduced with our latest branch cut changes and
should be fixed with this commit.

Signed-off-by: Sascha Grunert <[email protected]>

Author: saschagrunert

pkg/github/github.go

@@ -323,8 +323,8 @@ func (g *GitHub) LatestGitHubTagsPerBranch() (TagsPerBranch, error) {
 	for _, t := range allTags {
 		tag := t.GetName()
 
-		// Alpha releases are only available on the master branch
-		if strings.Contains(tag, "alpha") {
+		// alpha and beta releases are only available on the master branch
+		if strings.Contains(tag, "beta") || strings.Contains(tag, "alpha") {
 			releases.addIfNotExisting(git.Master, tag)
 			continue
 		}

pkg/github/github_test.go

@@ -125,9 +125,9 @@ func TestLatestGitHubTagsPerBranchSuccessMultipleForSameBranch(t *testing.T) {
 
 	// Then
 	require.Nil(t, err)
-	require.Len(t, res, 5)
-	require.Equal(t, tag2, res[git.Master])
-	require.Equal(t, tag1, res["release-1.18"])
+	require.Len(t, res, 4)
+	require.Equal(t, tag1, res[git.Master])
+	require.Empty(t, res["release-1.18"])
 	require.Empty(t, res["release-1.17"])
 	require.Equal(t, tag5, res["release-1.16"])
 	require.Equal(t, tag3, res["release-1.15"])