`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.

Start at the implementation and tests for `gh stack sync`; reproduce a sync where upstream changes exist and compare the current rebase behavior with a merge. Done means the command supports the requested non-rewriting behavior, with its option or default settled and tests covering the resulting history.

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