`gh stack submit` fails on forks by attempting to create PRs on upstream repository (`Head sha can't be blank`)

从 gh stack submit 入口开始，跟踪 repository 解析过程，包括 gh-stack.remote 和配置的默认 repository，直到 GraphQL createPullRequest 调用。使用 fork 设置和 GH_DEBUG=1 进行复现。完成标准是堆叠 PR 的创建目标为 fork，或明确支持预期的 head repository，且不再出现空 SHA 错误。

When using gh stack submit on a repository cloned from a fork, the command attempts to query and submit the stack against the upstream parent repository instead of the fork (or specified default repository).

Since the feature branches only exist on the fork (origin), GitHub's GraphQL API rejects the PR creation because the branches do not exist on the target upstream repository, resulting in Head sha can't be blank / Base sha can't be blank.

Executing gh repo set-default <fork-owner>/<repo> does not alter this behavior.

Environment

• OS: Windows 11 (Git Bash / MINGW64)
• gh version: gh version 2.100.0 (2026-09-03)
• gh stack extension version: github/gh-stack v0.1.1

Repository Setup

• origin: https://github.com/Hakzai/pokerpulsepro-tauri.git (Fork)
• upstream: https://github.com/davidelvar/pokerpulsepro-tauri.git (Parent Repo)
• gh repo set-default is explicitly set to Hakzai/pokerpulsepro-tauri.

Steps to Reproduce

1. Clone a fork repository that has an upstream remote configured.
2. Create stacked branches locally and push them to origin:
   a. main -> feature/branch-1 -> feature/branch-2
3. Set the default repository using gh repo set-default Hakzai/pokerpulsepro-tauri.
4. Run gh stack submit.

Expected Behavior

gh stack submit should resolve stacked PRs against the configured default repository (Hakzai/pokerpulsepro-tauri) or provide cross-repository fork support by specifying the head branch owner/ref.

Actual Behavior

The command targets davidelvar/pokerpulsepro-tauri and fails with:

⚠ failed to create PR for feature/add-brazil-real-currency: creating PR: GraphQL: Head sha can't be blank, Base sha can't be blank, No commits between main and feature/add-brazil-real-currency, Head ref must be a branch (createPullRequest)

Debug Output (GH_DEBUG=1 gh stack submit)

Relevant log excerpts showing API requests targeting the upstream repo instead of the fork:

* Request to https://api.github.com/repos/davidelvar/pokerpulsepro-tauri/stacks?per_page=100&page=1
...
[git.exe push origin --force-with-lease=refs/heads/feature/add-brazil-real-currency:... refs/heads/feature/add-brazil-real-currency:refs/heads/feature/add-brazil-real-currency]
* Request to https://api.github.com/graphql
⚠ failed to create PR for feature/add-brazil-real-currency: creating PR: GraphQL: Head sha can't be blank, Base sha can't be blank, No commits between main and feature/add-brazil-real-currency, Head ref must be a branch (createPullRequest)

Git Config

Git config is pointing correctly the upstream to davidelvar/pokerpulsepro-tauri and the origin to the fork at Hakzai/pokerpulsepro-tauri.

submodule.active=.
remote.origin.url=https://github.com/Hakzai/pokerpulsepro-tauri.git
remote.origin.fetch=+refs/heads/*:refs/remotes/origin/*
remote.origin.gh-resolved=base
branch.main.remote=origin
branch.main.merge=refs/heads/main
branch.main.vscode-merge-base=origin/main
lfs.repositoryformatversion=0
remote.upstream.url=https://github.com/davidelvar/pokerpulsepro-tauri.git
remote.upstream.fetch=+refs/heads/*:refs/remotes/upstream/*
...
branch.feature/full-backup-impl.vscode-merge-base=origin/main
branch.feature/full-backup-impl.remote=origin
branch.feature/full-backup-impl.merge=refs/heads/feature/full-backup-impl
branch.feature/add-brazil-real-currency.vscode-merge-base=origin/main
branch.feature/add-brazil-real-currency.remote=origin
branch.feature/add-brazil-real-currency.merge=refs/heads/feature/add-brazil-real-currency
branch.feature/create-winners-board.vscode-merge-base=origin/main
branch.feature/create-winners-board.remote=origin
branch.feature/create-winners-board.merge=refs/heads/feature/create-winners-board
branch.bugfix/prize-tab-not-showing-champion.vscode-merge-base=origin/main
branch.bugfix/prize-tab-not-showing-champion.remote=origin
branch.bugfix/prize-tab-not-showing-champion.merge=refs/heads/bugfix/prize-tab-not-showing-champion
branch.chore/deploy-to-github-pages.vscode-merge-base=origin/main
branch.chore/deploy-to-github-pages.remote=origin
branch.chore/deploy-to-github-pages.merge=refs/heads/chore/deploy-to-github-pages
rerere.enabled=true
rerere.autoupdate=true
gh-stack.remote=origin