Git merge master into orphan without commit history

Question

Given an orphan branch without history, how would I incorporate changes done on master branch, since the moment of creation of the orphan, without copying all commits history when pushing the orphan remotely.

A <- B <- C <- D  

        orphan(created from state B) <-X <- Y

I would like to bring c and d to the orphan branch and then x and y to master.

If I do checkout orphan, merge master or rebase master orphan gets the commits, but also all history tree. such as when I push orphan to a remote server, everybody will be able to see all master's history as well.

Also later I would like to merge orphan back into master, bringing commits x and y there.

Edit:

Now merging orphan into master works ok with git merge

A <- B <- C <- D <------X <- Y
                       /  
        orphan <-X <- Y 

But, merging master back into orphan either puts the entire master history into orphan (such as becomes X preceded by B) or with cherry picking, but than I need to skip the merge-commits and also get a lot more conflicts

Answer 1

Grafts are built for exactly this.

root=$(git rev-list my-orphan-branch --max-parents=0)  # get the orphaned-branch root
echo $(git rev-parse $root B) >.git/info/grafts        # locally remember its real parent

and now all the local commands will know about the ancestry but it will remain repo-local, push and fetch won't export it.