Insert commit as first, allowing rebase of initial commit

rebase_first_commit.sh

# first you need a new empty branch; let's call it `newroot`
git checkout --orphan newroot
git rm --cached -r .

# then you apply the same steps
git commit --allow-empty -m 'initial commit'
# or add some files for the first commit
git add .
git commit -m 'initial commit'

git rebase --onto newroot --root master
git branch -d newroot

# if you have another files is required to do this
git rebase --skip
# and finally
git push --force
git branch -D newroot