`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 😬

Start by tracing the `gh stack sync` command's prune confirmation output and the code that lists merged branches. Update the pre-confirmation message so it names each branch that will be pruned, then verify that the names shown before confirmation match the branches removed after approval.

索引モデルが issue の本文から書いたものです。