gh stack rebase replays the same child commit twice on v0.1.1

## Summary

`gh stack rebase` appears to replay the same child commit twice when a lower
branch in a stack receives a new commit.

This reproduces consistently for me with `gh stack` v0.1.1 on a freshly
created stack.

This looks similar to #250, but I am reproducing it on v0.1.1, which includes
the fix for amended parent commits being replayed into child branches.

## Environment

- OS: Windows
- Shell: PowerShell
- `gh stack version`: 0.1.1
- Stack trunk: `main`

## Stack

I created a simple three-layer stack:

main
└─ stack3/01-model
└─ stack3/02-service
└─ stack3/03-api

Each layer initially has one commit:

- `stack3/01-model`: Add Product model
- `stack3/02-service`: Add Product Service
- `stack3/03-api`: Add Product API

I then added another commit to the bottom layer (`stack3/01-model`).

Conceptually:

main
└─ Add Product model
└─ Add price to Product model
└─ [Product Service should be rebased here]
└─ [Product API should be rebased here]

## Reproduction

After adding the new commit to `stack3/01-model`, I ran:

gh stack rebase

Output:

Fetched latest main from origin
Trunk main is already up to date
Stack detected: (main) <- stack3/01-model <- stack3/02-service <- stack3/03-api
Rebasing branches in order, starting from stack3/01-model to stack3/03-api
Rebased stack3/01-model onto main

The command then stops while rebasing `stack3/02-service`.

From another terminal:

git status

reports:

interactive rebase in progress; onto 9c2c2c2

Last command done (1 command done):
pick 96bd07c # Add Product Service

Next command to do (1 remaining command):
pick 96bd07c # Add Product Service

You are currently rebasing branch 'stack3/02-service' on '9c2c2c2'.
(all conflicts fixed: run "git rebase --continue")

Changes to be committed:
new file: src/product-service.js

I inspected the rebase state.

`git rebase --edit-todo` contains:

pick 96bd07c # Add Product Service

And:

Get-Content .git\rebase-merge\done

contains:

pick 96bd07c... # Add Product Service

Therefore the same commit (`96bd07c`) appears to have already been processed
and is also still present as the next commit in the rebase todo.

## Actual behavior

The cascade rebase gets stuck while rebasing the second layer.

The same `Add Product Service` commit appears as both:

- the last completed rebase command, and
- the next remaining rebase command.

`src/product-service.js` is already staged from the first application.

## Expected behavior

Each layer's own commit range should be replayed exactly once onto the newly
rebased parent.

Conceptually, after changing the bottom layer:

main
└─ model
└─ model-update
└─ service'
└─ api'

The service commit should be replayed once onto the new `01-model` head, and
the API commit should then be replayed once onto the new `02-service` head.

## Additional notes

I previously encountered a similar failure with another stack. To rule out
corrupted state from that experiment, I created `stack3` from scratch and
reproduced the problem again.

I also confirmed that the installed extension is current:

gh stack -v
gh stack version 0.1.1

gh extension upgrade github/gh-stack
[stack]: already up to date

This looks related to #250, but that issue reproduced on v0.0.8 and the
release notes indicate that amended-parent replay behavior has since been
fixed.

Happy to provide the repository or additional `.git/gh-stack` / rebase
metadata if useful.

Start at the `gh stack rebase` command and reproduce the three-layer stack described in the issue on Windows or PowerShell. Inspect the cascade rebase state for `stack3/02-service`, including `.git/rebase-merge/done` and `git rebase --edit-todo`. Done means each layer's own commit range is replayed exactly once and the API layer rebases successfully onto the updated service layer.

Written by the indexing model from the issue text.