Suppose I have a branch A that I've been working on that corresponds to a merge request. A runs a few commits ahead of origin/A and someone else mistakenly merges this branch in. Now all the commits in origin/A have been flattened, and now I have to replay all these commits between origin/A and A onto a new branch created from master and submit a new MR for this. What's the right way to go about this? Manually I'd just cherry pick them over, but I have to imagine there's a more natural way to go about this.
Detect what was the last commit that was squash merged from your branch and let's call that commit X. Once you have that, you can run:
git rebase X my-branch --onto the-upstream-branch
That way you will only rebase commits that are after X.