Bump git2 from 0.16.1 to 0.18.1 (#575)

Bumps [git2](https://github.com/rust-lang/git2-rs) from 0.16.1 to 0.18.1.
- [Changelog](https://github.com/rust-lang/git2-rs/blob/master/CHANGELOG.md)
- [Commits](rust-lang/git2-rs@0.16.1...git2-0.18.1)

---
updated-dependencies:
- dependency-name: git2
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>