Branching from the wrong branch in git

branch_fix.sh

# branch/current <- Branch with commits
# branch/main    <- Branch that should have been branched from
# branch/other   <- Branch that was actually branched from

git checkout branch/main
git checkout -b branch/correct
git rebase --onto branch/correct branch/other branch/current

First, checkout the branch that you wanted to branch from originally.

Make a new branch, here we've called it branch/correct.

Rebase the changes made that are the difference between the branch you branched from and the branch you made with your latest commits.

Done.