It also depends on your company procedures.
All companies I know consider damp as dry for performance calculation, as long as there is no BC or BA (braking action or coefficient) is given. If ATC doesn't give you one of these two, you can safely assume that the BA is "good" and the BC is above 0.4.
It's something else if ATC tells you BC "unreliable", but mostly this is not because the runway is slippery, but because the BC is not measurable with the equipment given on the airport.
But it's also good to go on the safer side and do the divertion.
hth,
Dani