Add a way to merge pages by language

As an example:

```html
{{ $pages := .Site.RegularPages | lang.Merge $frSite.RegularPages | lang.Merge $enSite.RegularPages }}
```

Will "fill in the gaps" in the current site with, from left to right, content from the French site, and lastly the English.

Fixes #4463

hugolib/pages_language_merge_test.go

@@ -0,0 +1,150 @@
+// Copyright 2018 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 hugolib
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestMergeLanguages(t *testing.T) {
+	t.Parallel()
+	assert := require.New(t)
+
+	b := newTestSiteForLanguageMerge(t, 30)
+	b.CreateSites()
+
+	b.Build(BuildCfg{SkipRender: true})
+
+	h := b.H
+
+	enSite := h.Sites[0]
+	frSite := h.Sites[1]
+	nnSite := h.Sites[2]
+
+	assert.Equal(30, len(enSite.RegularPages))
+	assert.Equal(6, len(frSite.RegularPages))
+	assert.Equal(11, len(nnSite.RegularPages))
+
+	for i := 0; i < 2; i++ {
+		mergedNN := nnSite.RegularPages.MergeByLanguage(enSite.RegularPages)
+		assert.Equal(30, len(mergedNN))
+		for i := 1; i <= 30; i++ {
+			expectedLang := "en"
+			if i == 2 || i%3 == 0 {
+				expectedLang = "nn"
+			}
+			p := mergedNN[i-1]
+			assert.Equal(expectedLang, p.Lang(), fmt.Sprintf("Test %d", i))
+		}
+	}
+
+	mergedFR := frSite.RegularPages.MergeByLanguage(enSite.RegularPages)
+	assert.Equal(30, len(mergedFR))
+	for i := 1; i <= 30; i++ {
+		expectedLang := "en"
+		if i%5 == 0 {
+			expectedLang = "fr"
+		}
+		p := mergedFR[i-1]
+		assert.Equal(expectedLang, p.Lang(), fmt.Sprintf("Test %d", i))
+	}
+
+	firstNN := nnSite.RegularPages[0]
+	assert.Equal(4, len(firstNN.Sites()))
+	assert.Equal("en", firstNN.Sites().First().Language.Lang)
+
+}
+
+func TestMergeLanguagesTemplate(t *testing.T) {
+	t.Parallel()
+
+	b := newTestSiteForLanguageMerge(t, 15)
+	b.WithTemplates("home.html", `
+{{ $pages := .Site.RegularPages }}
+{{ .Scratch.Set "pages" $pages }}
+{{ if eq .Lang "nn" }}:
+{{ $enSite := index .Sites 0 }}
+{{ $frSite := index .Sites 1 }}
+{{ .Scratch.Set "pages" ($pages | lang.Merge $frSite.RegularPages| lang.Merge $enSite.RegularPages) }}
+{{ end }}
+{{ $pages := .Scratch.Get "pages" }}
+{{ range $i, $p := $pages }}{{ add $i 1 }}: {{ .Path }} {{ .Lang }} | {{ end }}
+`,
+		"shortcodes/shortcode.html", "MyShort",
+		"shortcodes/lingo.html", "MyLingo",
+	)
+
+	b.CreateSites()
+	b.Build(BuildCfg{})
+
+	b.AssertFileContent("public/nn/index.html", "p1.md en | 2: p2.nn.md nn | 3: p3.nn.md nn | 4: p4.md en | 5: p5.fr.md fr | 6: p6.nn.md nn | 7: p7.md en | 8: p8.md en | 9: p9.nn.md nn | 10: p10.fr.md fr | 11: p11.md en | 12: p12.nn.md nn | 13: p13.md en | 14: p14.md en | 15: p15.nn.md nn")
+}
+
+func newTestSiteForLanguageMerge(t testing.TB, count int) *sitesBuilder {
+	contentTemplate := `---
+title: doc%d
+weight: %d
+date: "2018-02-28"
+---
+# doc
+*some "content"*
+
+{{< shortcode >}}
+
+{{< lingo >}}
+`
+
+	builder := newTestSitesBuilder(t).WithDefaultMultiSiteConfig()
+
+	// We need some content with some missing translations.
+	// "en" is the main language, so add some English content + some Norwegian (nn, nynorsk) content.
+	var contentPairs []string
+	for i := 1; i <= count; i++ {
+		content := fmt.Sprintf(contentTemplate, i, i)
+		contentPairs = append(contentPairs, []string{fmt.Sprintf("p%d.md", i), content}...)
+		if i == 2 || i%3 == 0 {
+			// Add page 2,3, 6, 9 ... to both languages
+			contentPairs = append(contentPairs, []string{fmt.Sprintf("p%d.nn.md", i), content}...)
+		}
+		if i%5 == 0 {
+			// Add some French content, too.
+			contentPairs = append(contentPairs, []string{fmt.Sprintf("p%d.fr.md", i), content}...)
+		}
+	}
+
+	builder.WithContent(contentPairs...)
+	return builder
+}
+
+func BenchmarkMergeByLanguage(b *testing.B) {
+	const count = 100
+
+	builder := newTestSiteForLanguageMerge(b, count)
+	builder.CreateSites()
+	builder.Build(BuildCfg{SkipRender: true})
+	h := builder.H
+
+	enSite := h.Sites[0]
+	nnSite := h.Sites[2]
+
+	for i := 0; i < b.N; i++ {
+		merged := nnSite.RegularPages.MergeByLanguage(enSite.RegularPages)
+		if len(merged) != count {
+			b.Fatal("Count mismatch")
+		}
+	}
+}