`gh stack sync` should tell you what branches it wants to prune on your behalf

Here's a state where I just used `gh stack sync` after manually merging PR 2 in a stack of 7 PRs.

Everything looks good, except `Prune 1 merged branch` is a bit unclear.

I've already run `gh stack sync` after merging PR 1 too, so the branch for PR 1, `feature/sources-1-oak-cache` is already gone.

So the 1 branch to prune must be referring to `feature/sources-2-oak-source`, which makes sense, but it requires me to kind of interpret between the lines here.

It would be a bit more obvious (and safer?) if `Prune 1 merged branch` just listed the names of the branches it wants to prune somewhere. You see them after you say `Y`, but that's a bit too late 😬

Comece rastreando a saída de confirmação do prune do comando `gh stack sync` e o código que lista as branches mescladas. Atualize a mensagem exibida antes da confirmação para que ela nomeie cada branch que será removida e, em seguida, verifique se os nomes mostrados antes da confirmação correspondem às branches removidas após a aprovação.

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