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

## 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](https://github.slack.com/archives/C09M2GA1KQX/p1787881967913419?thread_ts=1787881967.913419&cid=C09M2GA1KQX)._

Reproduce the empty-diff stack in the issue, then trace the gh stack init and gh stack submit flows that create or submit PRs. Determine where a branch has no commits beyond its base and define whether the command should warn or stop. Done means the empty-diff case no longer gets silently auto-marked as merged, with coverage for the reported stack scenario.

Written by the indexing model from the issue text.