equalfile.go
func readPartial(c *Cmp, r io.Reader, buf []byte, n1, n2 int) (int, error) {
	for n1 < n2 {
		n, err := c.read(r, buf[n1:n2])
		n1 += n
		if err != nil {
			return n1, err
		}
	}
	return n1, nil
}
func readPartial(c *Cmp, r io.Reader, buf []byte, n1, n2 int) (int, error) {
	for n1 < n2 {
		n, err := c.read(r, buf[n1:n2])
		n1 += n
		if err != nil {
			return n1, err
		}
	}
	return n1, nil
}
// readPartial keeps reading from reader into provided buffer,
// until buffer size reaches exactly n2. n1 is initial buffer size.
// useful to ensure we get an specific buffer size from reader,
// withstanding partial reads.