utils/github: handle over 30 changed files

[cho-m]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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Related to issue seen while working on Homebrew/homebrew-core#198115 and temporary alternative CODEOWNERS in Homebrew/homebrew-core#201069

Before

brew(main):001> GitHub.get_pull_request_changed_files("Homebrew/homebrew-core", 198115).size
=> 30
brew(main):002> GitHub.get_pull_request_changed_files("Homebrew/homebrew-core", 198115).last["filename"]
=> "Formula/l/leela-zero.rb"

After

brew(main):001> GitHub.get_pull_request_changed_files("Homebrew/homebrew-core", 198115).size
=> 77
brew(main):002> GitHub.get_pull_request_changed_files("Homebrew/homebrew-core", 198115).last["filename"]
=> "Formula/z/znc.rb"

[MikeMcQuaid]

Thanks @carlocab!