[PATCH] changegroup: do not prompt to merge after adding only closed heads (issue2697)

[timeless]

This is getting long, but it seems like a case where there should be a
change table. Matt generally asks for them.

Offhand, I agree with Adrian, I had trouble reading the description,
and I think the message that would be displayed is still confusing.

I'm going to be traveling until I travel to the sprint, so I don't
really have much time to help with this one. Sorry.

fwiw, I do appreciate that someone pointed out that my change message
for my latest patch round was bad, it was.