Replace gh pr checkout with git fetch refs/pull to avoid GH_HOST issues

[dsyme]

Summary

Replaces gh pr checkout with pure git operations for PR checkout in checkout_pr_branch.cjs, eliminating the dependency on GH_HOST and avoiding DIFC proxy conflicts.

Problem

When the DIFC proxy is active, it sets GH_HOST=localhost:18443 which causes gh pr checkout to fail because the proxy address doesn't match any git remote. The previous fix (getGhEnvBypassingIntegrityFilteringForGitOps) worked around this by overriding GH_HOST per-call, but this is fragile and requires maintenance.

Solution

Use git fetch origin +refs/pull/N/head:refs/remotes/origin/pr-head followed by git checkout -B {branch} origin/pr-head instead of gh pr checkout. Git operations use remote URLs directly and are completely unaffected by GH_HOST.

GitHub exposes refs/pull/N/head for all PRs, including fork PRs, so this works universally.

Changes

• checkout_pr_branch.cjs: Replace gh pr checkout with git fetch refs/pull/N/head + git checkout -B. Fetches the correct depth upfront via the API (no separate history-deepening step needed). Remove git_helpers.cjs import.
• git_helpers.cjs: Remove getGitHubHost() and getGhEnvBypassingIntegrityFilteringForGitOps() — no consumers remain.
• checkout_pr_branch.test.cjs: Update all 47 tests to validate the new git-based checkout flow rather than gh pr checkout. Remove the GH_HOST override test group.

Testing

All 47 checkout tests + 19 git_helpers tests pass.

[Copilot]

Pull request overview

This PR removes reliance on gh pr checkout for checking out pull request branches and replaces it with a pure-git flow (git fetch origin +refs/pull/N/head:... + git checkout -B ...), avoiding failures caused by GH_HOST overrides (e.g., DIFC proxy).

Changes:

• Replace gh pr checkout with git fetch origin +refs/pull/N/head and git checkout -B in checkout_pr_branch.cjs.
• Remove now-unused GH_HOST/host-derivation helpers from git_helpers.cjs.
• Update the checkout test suite to assert the new git-based behavior and drop GH_HOST-override coverage.

Show a summary per file

File	Description
actions/setup/js/checkout_pr_branch.cjs	Switch checkout strategy from gh CLI to fetching refs/pull/N/head and checking out a local branch from the fetched ref.
actions/setup/js/git_helpers.cjs	Remove GH_HOST/GHE host derivation helpers that no longer have consumers.
actions/setup/js/checkout_pr_branch.test.cjs	Update tests to validate the new fetch-by-ref checkout flow and remove GH_HOST override expectations.

Copilot's findings

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Comments suppressed due to low confidence (2)

actions/setup/js/checkout_pr_branch.cjs:180

• The else-branch now always calls the GitHub API (fetchPRDetails) to get commitCount and headRef, even when the event payload already contains pullRequest.commits and pullRequest.head.ref (e.g. pull_request_target, pull_request fork payloads). This adds avoidable latency and increases the chance of rate-limit / transient API failures. Prefer using the payload values when present, only falling back to the API when the data is missing (like issue_comment).

      // Get PR details from API to determine head ref name and commit count
      const { commitCount, headRef } = await fetchPRDetails(prNumber);
      const fetchDepth = (commitCount || 1) + 1; // +1 to include the merge base

actions/setup/js/checkout_pr_branch.cjs:187

• git checkout -B ${branchName} origin/pr-head force-resets/overwrites any existing local branch with the same name. On persistent workspaces (e.g. self-hosted runners or scripts re-run in the same directory) this can clobber local state unexpectedly. Consider always checking out to a unique, namespaced branch (e.g. pr-${prNumber}) and/or avoiding -B unless you explicitly want to reset an existing branch.

      const branchName = headRef || `pr-${prNumber}`;
      core.info(`Checking out branch: ${branchName}`);
      await exec.exec("git", ["checkout", "-B", branchName, "origin/pr-head"]);

• Files reviewed: 3/3 changed files
• Comments generated: 2