Automate the Hugo release process

This commit adds a work flow aroung GoReleaser to get the Hugo release process automated and more uniform:

* It can be run fully automated or in two steps to allow for manual edits of the relase notes.
* It supports both patch and full releases.
* It fetches author, issue, repo info. etc. for the release notes from GitHub.
* The file names produced are mainly the same as before, but we no use tar.gz as archive for all Unix versions.
* There isn't a fully automated CI setup in place yet, but the release tag is marked in the commit message with "[ci deploy]"

Fixes #3358

releaser/releasenotes_writer_test.go

@@ -0,0 +1,44 @@
+// Copyright 2017-present The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package commands defines and implements command-line commands and flags
+// used by Hugo. Commands and flags are implemented using Cobra.
+
+package releaser
+
+import (
+	"bytes"
+	"fmt"
+	"testing"
+
+	"runtime"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestReleaseNotesWriter(t *testing.T) {
+	if runtime.GOOS == "linux" {
+		// Travis has an ancient git with no --invert-grep: https://github.com/travis-ci/travis-ci/issues/6328
+		t.Skip("Skip git test on Linux to make Travis happy.")
+	}
+
+	var b bytes.Buffer
+
+	// TODO(bep) consider to query GitHub directly for the gitlog with author info, probably faster.
+	infos, err := getGitInfosBefore("v0.20", false)
+	require.NoError(t, err)
+
+	require.NoError(t, writeReleaseNotes("0.20", infos, &b))
+
+	fmt.Println(">>>", b.String())
+}