bytes: don't grow Buffer if capacity is available

Also added a new benchmark from the same test:

benchmark                           old ns/op    new ns/op    delta
BenchmarkBufferNotEmptyWriteRead      2643698       709189  -73.17%

Fixes #5154

R=golang-dev, r, gri
CC=golang-dev
https://golang.org/cl/8164043

src/pkg/bytes/export_test.go

@@ -7,3 +7,7 @@ package bytes
 // Export func for testing
 var IndexBytePortable = indexBytePortable
 var EqualPortable = equalPortable
+
+func (b *Buffer) Cap() int {
+	return cap(b.buf)
+}