gh stack modify: modify fails when merged branches have been pruned

Looks like modify can't deal with pruned branches once they have been merged.

See below output:

```
➜ token-scanning-service git:(aerendel/fix-generic-secrets-metadata) gh stack modify
✗ failed to apply modifications: building snapshot: resolving branch SHAs: failed to run git: fatal: ambiguous argument 'aerendel/mixed-content-types1': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git [...] -- [...]'
```

### Observed behavior:
The bottom of my stack has been merged and the branch pruned so it doesn't exist locally or remotely anymore (not unusual once a branch is merged), and all `gh stack modify` operations fail to save. Any operation will fail, wether it's renaming a branch, reorganizing, dropping a branch, etc.

### Repro:
1. Make a stack with 2 or 3 layers
2. Merge the first layer
3. Delete the first layer's branch locally (or just answer Yes to gh stack's prune prompt)
4. Run `gh stack modify` and try to save any modify operation

gh stack modify フローと、ブランチの SHA を解決する snapshot ステップから始め、2 層または 3 層のスタックの最下層のブランチを削除して問題を再現します。マージされたブランチがローカルとリモートから削除された後でも、名前変更、再編成、削除、その他の modify 操作を正常に保存できれば完了です。

索引モデルが issue の本文から書いたものです。