fetch-depth: 0 no longer sets up origin/HEAD after Git 2.50 update

[idan-rahamim-lendbuzz]

Issue Description

Summary

Following the update of the ubuntu24/20250710.1 runner image—which included upgrading Git from 2.49.0 to 2.50.1—using actions/checkout@v4 with fetch-depth: 0 no longer sets up the origin/HEAD symbolic reference. As a result, workflows that rely on origin/HEAD..HEAD as a valid revision range are now broken.

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Steps to Reproduce

1. Use actions/checkout@v4 with fetch-depth: 0 on an ubuntu-latest runner.
2. Execute a command referencing origin/HEAD (e.g., git diff origin/HEAD..HEAD).
3. Observe the following error:

   fatal: ambiguous argument 'origin/HEAD..HEAD': unknown revision or path not in the working tree
   

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Expected Behavior

• The origin/HEAD symbolic ref should automatically point to the default branch, mirroring the behavior of a standard git clone.

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Actual Behavior

• The origin/HEAD reference is missing, causing failures in tools and scripts that rely on it (for example, pre-commit when invoked with --from-ref origin/HEAD --to-ref HEAD).

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Environment

• Runner: ubuntu-latest (ubuntu24/20250710.1)
• Git version: 2.50.1 (previously 2.49.0)
• Checkout version: actions/checkout@v4

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Example Workflow

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
        with:
          fetch-depth: 0
      - run: git diff origin/HEAD..HEAD

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Workaround

Adding the following step immediately after checkout restores the expected behavior:

- name: Set up git references
  run: git remote set-head origin --auto

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Root Cause

Git 2.50 introduced changes to default branch handling during clone operations, which appears to affect how origin/HEAD is configured.

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Request

Please update actions/checkout to ensure that origin/HEAD is correctly set when fetch-depth: 0 is used, maintaining compatibility with workflows that depend on this reference.