Various issues exhibited when local and remote base branch names are not the same

Due to having a single repo with many different forks of LLVM checked out into various worktrees, my local branch names for things like "main" are often prefixed; e.g. my local branch that tracks llvm/llvm-project is upstream-main. For most operations this seems to work fine, I could gh stack init --base upstream-main and create my stack locally. However, when it came time to submit, I got the following:

```
% gh stack submit --auto --open
Checking stack state...
? Multiple remotes found. Which remote should be used? upstream
? Save "upstream" as the default remote for all gh stack operations? No
Pushing to upstream...
⚠ failed to create PR for users/jrtc27/lld-test-aarch64-ifunc-bti: creating PR: GraphQL: Head sha can't be blank, Base sha can't be blank, No commits between upstream-main and users/jrtc27/lld-test-aarch64-ifunc-bti, Base ref must be a branch (createPullRequest)
✓ Created PR #210611 for users/jrtc27/lld-test-ppc32-ifunc-nonpreemptible
✓ Created PR #210612 for users/jrtc27/lld-replaceWithDefined-skip-copy
✓ Created PR #210613 for users/jrtc27/lld-handleNonPreemptibleIfunc-skip-pointless-alias
✓ Created PR #210614 for users/jrtc27/lld-handleNonPreemptibleIfunc-no-copy-constructor
✓ Created PR #210615 for users/jrtc27/lld-Symbol-remove-copy-constructor
✓ Created PR #210616 for users/jrtc27/lld-handleNonPreemptibleIfunc-addSyntheticLocal
✓ Stack created on GitHub with 6 PRs (stack #210617)
✓ Pushed and synced 7 branches
```

There are three issues here:

1. gh-stack should have been able to figure out that, given upstream-main is the base branch tracking refs/remotes/upstream/main, upstream is the sensible remote to automatically use (but that's a minor nice-to-have)
2. gh-stack should be querying the upstream branch for the base branch and using that as the branch name (minus the refs/remotes prefix), not the local branch (presumably only falling back to the local branch name if there is no upstream branch, if that's a sensible thing to do)
3. If creating a PR fails, gh-stack should stop before it makes things worse, as fixing this up manually is a bit of a pain (you have to create the missing PR, unstack the existing stack and only then can you re-stack the entire thing, since gh stack link currently doesn't let you add to the front of the stack, presumably due to the API being a bit deficient...)

Comienza con el flujo gh stack submit --auto --open y sigue cómo se seleccionan las ramas locales, las refs de seguimiento de upstream y los remotos antes de crear el pull request. Reproduce el caso upstream-main informado y verifica la rama base enviada a GitHub, la selección del remoto y el comportamiento después de un fallo en la creación de la PR. Se considera terminado cuando los tres problemas informados están resueltos sin dejar un stack enviado parcialmente.

Escrito por el modelo de indexación a partir del texto del issue.