##git merge –strategy=theirs (closest we can get)
get a temp copy of the branch you want to 'clone' into another
git checkout -b tmp <branch to copy from>
merge our version of the branch into the upstream with ours strategy
git merge --strategy=ours <branch to paste into>
checkout our version of the branch
git checkout <branch to paste into>
merge temp (which will be a fast forward)
git merge tmp
get rid of the temp branch
git branch -D temp
You don't need the intermediate branch, you can reset --hard the destination (current) branch taking the incoming (theirs) branch as the reset target. Then you can do a merge with the upstream branch with -s ours. I've learned this here.