bytes, strings: use copy in Repeat

R=golang-dev, dave, bradfitz, adg
CC=golang-dev
https://golang.org/cl/13249043

src/pkg/bytes/bytes.go

@@ -375,10 +375,7 @@ func Repeat(b []byte, count int) []byte {
 	nb := make([]byte, len(b)*count)
 	bp := 0
 	for i := 0; i < count; i++ {
-		for j := 0; j < len(b); j++ {
-			nb[bp] = b[j]
-			bp++
-		}
+		bp += copy(nb[bp:], b)
 	}
 	return nb
 }