Convert hsv to rgb.

Here is a little test case.

    (let ((color-name (format "#%06x" (random #xffffff))))
      (format "color-name: %s  round-tripped: %s"
              color-name
              (apply 'color-rgb-to-hex 
                     (apply 'color-hsv-to-rgb 
                            (apply 'color-rgb-to-hsv 
                                   (color-name-to-rgb color-name))))))

    => "color-name: #01043d  round-tripped: #01033d"
    => "color-name: #1278e2  round-tripped: #1278e2"
    => "color-name: #ffb4bd  round-tripped: #ffb4bd"

One can use `color-hsv-to-rgb' for example in `vc-annotate-color-map'.