Re: make add_paths_to_append_rel aware of startup cost

But overall, I'm more inclined to just go with the more simple "add a
cheap unordered startup append path if considering cheap startup
plans" version. I see your latest patch does both. So, I'd suggest two
patches as I do see the merit in keeping this simple and cheap.  If we
can get the first part in and you still find cases where you're not
getting the most appropriate startup plan based on the tuple fraction,
then we can reconsider what extra complexity we should endure in the
code based on the example query where we've demonstrated the planner
is not choosing the best startup path appropriate to the given tuple
fraction.