FR: support for cross-fork stacked PRs

The limitation that the source and target branch need to be on the same repository is a showstopper for my project's use of stacked PRs, and I expect it will similarly be so for almost any github project of nontrivial size. It's unreasonable to expect projects to grant all potential contributors branch creation permission, and my project would similarly consider it unreasonable to restrict stacked PR support to just some subset of trusted developers -- we don't want some developers to feel "second-class" to the extent we can avoid it.

I can understand how this restriction would arise -- presumably the repository associated with a PR is always the repository of the base branch, which requires the base branch (the previous PR in the stack) to be in the target repository for a stacked PR. But please consider ways in which it could be lifted, as the current limitation is a severe blocker to usage of this functionality.

Nenhum arquivo de código-fonte ou teste é mencionado na issue. Comece rastreando como gh-stack associa um pull request ao seu repositório base e valida branches empilhadas entre forks. Considera-se concluído quando um contribuidor pode criar e usar um PR empilhado cujas branches de origem e de destino estejam em repositórios diferentes, sem precisar de permissão para criar branches no repositório de destino.

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