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

Comece pelo fluxo gh stack modify e pela etapa de snapshot que resolve os SHAs das branches; em seguida, reproduza o problema removendo a branch inferior de uma stack de duas ou três camadas. O trabalho estará concluído quando operações de renomear, reorganizar, remover e outras operações de modificação puderem ser salvas com sucesso depois que a branch mesclada for excluída localmente e remotamente.

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