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.

No source files or tests are named in the issue. Start by tracing how gh-stack associates a pull request with its base repository and validates stacked branches across forks. Done means a contributor can create and use a stacked PR whose source and target branches are on different repositories without requiring branch creation permission in the target repository.

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