Fix linkage check script to handle detached HEAD state

[zhan7236]

The beam-linkage-check.sh script fails to restore the original git position when starting from a detached HEAD state. This commonly happens when verifying someone else's PR, which involves merging two non-branch references.

Root Cause:
The script used git rev-parse --abbrev-ref HEAD which returns the string "HEAD" in detached state instead of a valid reference. The cleanup function then fails to restore the original position.

Fix:

• Use git rev-parse HEAD to get the full commit SHA instead of branch name
• Add -c advice.detachedHead=false to suppress warnings during cleanup checkout

Fixes #20558

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[gemini-code-assist]

Summary of Changes

Hello @zhan7236, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a bug in the beam-linkage-check.sh script where it failed to correctly restore the Git repository's state after execution, specifically when the repository was in a detached HEAD state. The changes ensure that the script can reliably return to the original Git position without issues or warnings, improving the developer experience when verifying pull requests.

Highlights

• Fix beam-linkage-check.sh in detached HEAD state: The script previously failed to restore the original git position when the repository was in a detached HEAD state, which commonly occurs when verifying pull requests.
• Use full commit SHA for STARTING_REF: Changed git rev-parse --abbrev-ref HEAD to git rev-parse HEAD to correctly capture the full commit SHA, ensuring proper restoration of the git state.
• Suppress detached HEAD warnings: Added -c advice.detachedHead=false to the git checkout command in the cleanup function to prevent warnings when returning from a detached HEAD state.

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