Auto merge of #6438 - rust-lang:dependabot/cargo/git2-0.8.0, r=alexcrichton

Auto merge of #6438 - rust-lang:dependabot/cargo/git2-0.8.0, r=alexcrichton

Update git2 requirement from 0.7.5 to 0.8.0

Updates the requirements on [git2](https://github.com/alexcrichton/git2-rs) to permit the latest version.
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<summary>Commits</summary>

- See full diff in [compare view](https://github.com/alexcrichton/git2-rs/commits/git2-curl-0.8.0)
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