`gh stack sync` always rebases, which breaks PR review history

If a reviewer has already viewed/reviewed a PR, and the submitter performs a `gh stack sync`, it will then force-push, which erases useful information about "changes since last view" in the PR interface.

It should be an option (the default?) to perform a merge from upstream rather than a rebase to avoid messing up the review history.

從 `gh stack sync` 的實作和測試著手；重現 upstream 存在變更時的同步，並將目前的 rebase 行為與 merge 進行比較。完成標準是：該命令支援所要求的非重寫行為，確定透過選項還是預設行為實現，並且測試涵蓋由此產生的歷史記錄。

由索引模型根據 Issue 內容生成。