bytes: removed unnecessary slicing on copy

Change-Id: Ia42e3479c852a88968947411de8b32e5bcda5ae3
Reviewed-on: https://go-review.googlesource.com/64371Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/bytes/buffer.go

@@ -131,7 +131,7 @@ func (b *Buffer) grow(n int) int {
 		// slice. We only need m+n <= c to slide, but
 		// we instead let capacity get twice as large so we
 		// don't spend all our time copying.
-		copy(b.buf[:], b.buf[b.off:])
+		copy(b.buf, b.buf[b.off:])
 	} else if c > maxInt-c-n {
 		panic(ErrTooLarge)
 	} else {