perf: Remove need for fetching repository by using GitHub API in skip merge queue check

[Mrtenz]

This optimises the action to check if a pull request in the merge queue can be skipped by removing the need for actions/checkout with fetch-depth: 0. Instead of fetching the full history and using local git commands, we can use the GitHub API /compare endpoint, which returns a status based on whether the pull request is up-to-date or not.

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Note

Optimizes check-skip-merge-queue composite action to avoid full repo fetch and local git.

• Replaces actions/checkout + shell git merge-base with actions/github-script using the GitHub /compare (compareCommitsWithBasehead) API to determine if PR_BRANCH is identical/ahead of BASE_REF
• Adds early exit when merge queue position ≠ 1 and sets up-to-date output accordingly
• Keeps existing outputs while simplifying logic within action.yml

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[cryptodev-2s]

LGTM!

[FrederikBolding]

In what case would it return ahead versus identical?

[Mrtenz]

• identical: The base and head point to the same commit. Not sure when this would be the case, but it's safe to treat pull requests as up-to-date in this case.
• ahead: The head includes all commits of the base, plus additional commits.
• behind: The head is missing commits from the base.
• diverged: Both the head and base contain additional commits compared to the merge base.