text/template: replace bytes.Buffer with strings.Builder

After Go.1.10+ strings.Builder is known as more efficient in
concatenating and building strings than bytes.Buffer.

In this CL,
there is a minor logic fix for getting advantage of strings.builder.

name               old time/op    new time/op    delta
DefinedTemplate-8     543ns ± 3%     512ns ± 2%   -5.73%  (p=0.000 n=8+8)

name               old alloc/op   new alloc/op   delta
DefinedTemplate-8      192B ± 0%      160B ± 0%  -16.67%  (p=0.000 n=8+8)

name               old allocs/op  new allocs/op  delta
DefinedTemplate-8      5.00 ± 0%      5.00 ± 0%     ~     (all equal)

Change-Id: Icda0054d146e6c5e32ed8a4d13221bb6850d31b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/175261
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/text/template/exec.go

@@ -5,7 +5,6 @@
 package template
 
 import (
-	"bytes"
 	"fmt"
 	"internal/fmtsort"
 	"io"
@@ -230,21 +229,19 @@ func (t *Template) DefinedTemplates() string {
 	if t.common == nil {
 		return ""
 	}
-	var b bytes.Buffer
+	var b strings.Builder
 	for name, tmpl := range t.tmpl {
 		if tmpl.Tree == nil || tmpl.Root == nil {
 			continue
 		}
-		if b.Len() > 0 {
+		if b.Len() == 0 {
+			b.WriteString("; defined templates are: ")
+		} else {
 			b.WriteString(", ")
 		}
 		fmt.Fprintf(&b, "%q", name)
 	}
-	var s string
-	if b.Len() > 0 {
-		s = "; defined templates are: " + b.String()
-	}
-	return s
+	return b.String()
 }
 
 // Walk functions step through the major pieces of the template structure,