Detect and prevent empty-diff PRs from being auto-marked as merged during stack submit

重現 issue 中的 empty-diff stack，接著追蹤建立或提交 PRs 的 gh stack init 和 gh stack submit 流程。確認 branch 沒有超出其 base 的 commit 的位置，並定義 command 應該發出警告還是停止。當 empty-diff 情況不再被靜默地自動標記為 merged，且已涵蓋回報的 stack 情境時，即表示完成。

Summary

When using gh stack, it's possible to end up with a PR whose head branch has no commits beyond its base branch (i.e., an empty diff). When this happens, GitHub automatically marks that PR as "merged" as soon as it is synchronized (e.g., via gh stack submit pushing to origin), even though no actual merge was intended.

Repro scenario

Given a stack main <- branch A <- branch B:

1. Commit to A (intending it for B).
2. Check out B from A without making a new commit.
3. Push B and open PRs for A and B.
4. Run gh stack init to adopt the two branches into a stack.
5. Run gh stack submit.

Because B's branch contains no commits beyond A, GitHub treats the PR for B as having "no diff" against its base, and the subsequent push/synchronize causes GitHub to automatically mark PR B as merged into A — even though the user never intended or triggered a merge.

Observed example

• https://github.com/github/mdash-connector/pull/14
• https://github.com/github/mdash-connector/pull/15

After running gh stack init followed by gh stack submit, PR #15 (branch B) was unexpectedly shown as merged into PR #14 (branch A), because branch B had no unique commits relative to branch A.

Request

gh stack should proactively detect this situation (a branch in the stack with no commits ahead of its base) and warn or prevent the user from creating/submitting a PR in that state, rather than letting it silently get auto-merged by GitHub.

Context

Reported by @jschnapper, triaged by @skarim, in a Slack thread.

Created from a Slack conversation.