Fix GitHub API last commit and short commit check

[cho-m]

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• multiple_short_commits_exist? seems to be checking for some nonexistent string. Instead just put the http_code on stdout.
• last_commit curl output is now lowercase so just do case-insensitive match. If this breaks again, can consider using API.open_rest instead.
• last_commit was also outputting a full commit which doesn't work well with other parts of brew. GitHub API does not provide a good way to get the short commit so just attempt a guess at 7 characters.
• For GitHub download strategy, handle the case where last_commit fails and the repository isn't locally available

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I think this should help with:

• https://github.com/orgs/Homebrew/discussions/6512#discussioncomment-14916071
• Display correct outdated head information #20940 (review)

[Copilot]

Pull Request Overview

This PR improves the handling of Git commit hashes from the GitHub API by standardizing on 7-character short commits and adding better validation and ambiguity detection. The key changes include truncating commit hashes to 7 characters to match GitHub's GraphQL abbreviatedOid format, checking for ambiguous short commits using the GitHub API, and refactoring the caching logic in the download strategy to avoid caching empty strings when repositories haven't been fetched.

Key Changes

• Modified GitHub.last_commit to truncate commit hashes to 7 characters and verify they are unambiguous
• Updated GitHub.multiple_short_commits_exist? to use curl's HTTP status code output for more reliable ambiguity detection
• Refactored GitHubGitDownloadStrategy.last_commit caching logic to separate GitHub API and local git command results

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
Library/Homebrew/utils/github.rb	Added commit hash truncation to 7 characters, case-insensitive ETag regex, ambiguity checking, and improved HTTP status code detection in multiple_short_commits_exist?
Library/Homebrew/download_strategy.rb	Refactored last_commit method to avoid caching empty strings and added fallback logic in commit_outdated? for empty commit values

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[Copilot]

Pull Request Overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

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Pull Request Overview

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[MikeMcQuaid]

Thanks @cho-m! Some minor tweaks then good to ship without a re-review.

[Copilot]

Pull Request Overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

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[MikeMcQuaid]

@cho-m thanks for quick updates!