Rough sketch on how to turn a Git branch into an orphan branch

make-orphan.sh

branch=... # the branch you want to turn into an orphan branch
base=master # the branch which $branch is based on and shouldn’t be anymore
firstCommit=... # the first commit on $branch that’s not on $base

# rename $branch to $branch-bak
git checkout $branch
git checkout -b $branch-bak
git branch -d $branch

# new orphan $branch
git checkout --orphan $branch
git rm -rf .

# create a first commit that we can rebase on (git rebase --onto refuses to operate on the orphan branch)
git cherry-pick $firstCommit

# move all the other commits over
git rebase --onto $branch $(git merge-base $branch-bak $base) $branch-bak

# that leaves us on a detached head for whatever reason, so change $branch to point to that commit
branchSha=$(git show --no-patch --pretty=format:%H)
git checkout $branch
git reset --hard $branchSha

Line 6+7 could read
git checkout -f -b "$branch-bak" "$branch"

Line 8 should be
git branch -D "$branch"
to force deletion if unmerged (which $branch likely is)

Not sure on the git rebase --onto= site of things, why it doesn't update $branch