Ok, I’m asuming the branch is called “feature” and it was branched from “master”.
There’s this little git command called merge-base. It takes two commits and gives you the first common ancestor of both of those. So…
git merge-base feature master
…will give you the first common ancestor of those two commits. Guess what happens when you pass that commit to git rebase -i, like…
git rebase -i `git merge-base feature master`
Interactive rebase from the first common ancestor of both master and feature branch.
Profit! 😉