hugolib: Avoid recloning of shortcode templates

```bash
benchmark                                    old ns/op     new ns/op     delta
BenchmarkSiteNew/Bundle_with_image-4         14572242      14382188      -1.30%
BenchmarkSiteNew/Bundle_with_JSON_file-4     13683922      13738196      +0.40%
BenchmarkSiteNew/Multiple_languages-4        41912231      25192494      -39.89%

benchmark                                    old allocs     new allocs     delta
BenchmarkSiteNew/Bundle_with_image-4         57496          57493          -0.01%
BenchmarkSiteNew/Bundle_with_JSON_file-4     57492          57501          +0.02%
BenchmarkSiteNew/Multiple_languages-4        242422         118809         -50.99%

benchmark                                    old bytes     new bytes     delta
BenchmarkSiteNew/Bundle_with_image-4         3845077       3844065       -0.03%
BenchmarkSiteNew/Bundle_with_JSON_file-4     3627442       3627798       +0.01%
BenchmarkSiteNew/Multiple_languages-4        13963502      7543885       -45.97%
```

Fixes #5890

2 files changed, 125 insertions, 1 deletions

diff --git a/hugolib/site_benchmark_new_test.go b/hugolib/site_benchmark_new_test.go
new file mode 100644
index 000000000..c816dc9c3
--- /dev/null
+++ b/hugolib/site_benchmark_new_test.go
@@ -0,0 +1,106 @@
+// Copyright 2019 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 (
+	"testing"
+)
+
+// TODO(bep) eventually remove the old (too complicated setup).
+func BenchmarkSiteNew(b *testing.B) {
+	// TODO(bep) create some common and stable data set
+
+	const pageContent = `---
+title: "My Page"
+---
+
+My page content.
+
+`
+
+	config := `
+baseURL = "https://example.com"
+
+`
+
+	benchmarks := []struct {
+		name   string
+		create func(i int) *sitesBuilder
+		check  func(s *sitesBuilder)
+	}{
+		{"Bundle with image", func(i int) *sitesBuilder {
+			sb := newTestSitesBuilder(b).WithConfigFile("toml", config)
+			sb.WithContent("content/blog/mybundle/index.md", pageContent)
+			sb.WithSunset("content/blog/mybundle/sunset1.jpg")
+
+			return sb
+		},
+			func(s *sitesBuilder) {
+				s.AssertFileContent("public/blog/mybundle/index.html", "/blog/mybundle/sunset1.jpg")
+				s.CheckExists("public/blog/mybundle/sunset1.jpg")
+
+			},
+		},
+		{"Bundle with JSON file", func(i int) *sitesBuilder {
+			sb := newTestSitesBuilder(b).WithConfigFile("toml", config)
+			sb.WithContent("content/blog/mybundle/index.md", pageContent)
+			sb.WithContent("content/blog/mybundle/mydata.json", `{ "hello": "world" }`)
+
+			return sb
+		},
+			func(s *sitesBuilder) {
+				s.AssertFileContent("public/blog/mybundle/index.html", "Resources: application/json: /blog/mybundle/mydata.json")
+				s.CheckExists("public/blog/mybundle/mydata.json")
+
+			},
+		},
+		{"Multiple languages", func(i int) *sitesBuilder {
+			sb := newTestSitesBuilder(b).WithConfigFile("toml", `
+baseURL = "https://example.com"
+
+[languages]
+[languages.en]
+weight=1
+[languages.fr]
+weight=2
+			
+`)
+
+			return sb
+		},
+			func(s *sitesBuilder) {
+
+			},
+		},
+	}
+
+	for _, bm := range benchmarks {
+		b.Run(bm.name, func(b *testing.B) {
+			sites := make([]*sitesBuilder, b.N)
+			for i := 0; i < b.N; i++ {
+				sites[i] = bm.create(i)
+			}
+
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				s := sites[i]
+				err := s.BuildE(BuildCfg{})
+				if err != nil {
+					b.Fatal(err)
+				}
+				bm.check(s)
+			}
+		})
+	}
+}
diff --git a/hugolib/testhelpers_test.go b/hugolib/testhelpers_test.go
index d7c972443..fe0f824f2 100644
--- a/hugolib/testhelpers_test.go
+++ b/hugolib/testhelpers_test.go
@@ -1,6 +1,7 @@
 package hugolib
 
 import (
+	"io"
 	"io/ioutil"
 	"path/filepath"
 	"runtime"
@@ -42,6 +43,8 @@ type sitesBuilder struct {
 	Fs  *hugofs.Fs
 	T   testing.TB
 
+	*require.Assertions
+
 	logger *loggers.Logger
 
 	dumper litter.Options
@@ -88,7 +91,7 @@ func newTestSitesBuilder(t testing.TB) *sitesBuilder {
 		Separator:         " ",
 	}
 
-	return &sitesBuilder{T: t, Fs: fs, configFormat: "toml", dumper: litterOptions}
+	return &sitesBuilder{T: t, Assertions: require.New(t), Fs: fs, configFormat: "toml", dumper: litterOptions}
 }
 
 func createTempDir(prefix string) (string, func(), error) {
@@ -260,6 +263,21 @@ lag = "lag"
 
 }
 
+func (s *sitesBuilder) WithSunset(in string) {
+	// Write a real image into one of the bundle above.
+	src, err := os.Open(filepath.FromSlash("testdata/sunset.jpg"))
+	s.NoError(err)
+
+	out, err := s.Fs.Source.Create(filepath.FromSlash(in))
+	s.NoError(err)
+
+	_, err = io.Copy(out, src)
+	s.NoError(err)
+
+	out.Close()
+	src.Close()
+}
+
 func (s *sitesBuilder) WithContent(filenameContent ...string) *sitesBuilder {
 	s.contentFilePairs = append(s.contentFilePairs, filenameContent...)
 	return s