stck sync fails when parent branch was squash-merged

[Reinand]

Problem

When a parent branch in a stack is squash-merged into main, stck sync cannot rebase the remaining stack. Each original commit from the squash-merged branch conflicts with what's already in main, because git doesn't recognize them as already-upstream.

Steps to reproduce

1. Have a stack: main <- A <- B <- C
2. Branch A has commits a1, a2, a3
3. Squash-merge A into main (creates single commit a_squashed)
4. Retarget B to main
5. Run stck sync
6. Result: conflict on every commit from A (a1, a2, a3) because git doesn't know they're already in main

Workaround

We had to manually identify the fork point (last commit unique to the parent branch) and use git rebase --onto per branch:

git rebase --onto origin/main <old-parent-tip> <branch>

This had to be done for each branch in the stack, finding the correct fork point each time.

Suggestion

stck sync could detect squash-merged parents by comparing the tree/diff rather than commit hashes. Alternatively, it could use git rebase --onto with the merge-base of the old parent, skipping commits that are already represented in the target.

GitHub's "Squash and merge" is the most common merge strategy for PRs, so this is likely the primary scenario for stacked PR workflows.

[Reinand]

Real-world context

We hit this with a 6-branch stack where the first branch (enum refactor, ~8 commits across multiple files) was squash-merged into main. The remaining 5 branches all carried those 8 commits in their history.

stck sync hit a conflict on the very first commit and stopped. Even after resolving that conflict and continuing, the next commit conflicted too — this repeated for every original commit from the squash-merged branch.

The manual fix required finding the unique fork point for each of the 5 branches and running:

git rebase --onto <new-parent> <old-parent-tip> <branch>

This took significant effort to get right, especially identifying the correct <old-parent-tip> for each branch.

This is probably the most important issue to solve for stck to be usable in teams that use GitHub's default "Squash and merge" strategy.