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

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)
+			}
+		})
+	}
+}