git-branchless: update to use libgit2 1.9

[chenrui333]

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• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
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• build: bump git2 0.19 -> 0.20 to unlock ^ref spec arxanas/git-branchless#1485

[github-actions]

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