build(deps): bump pygit2 in /scripts/git-review-rebase

Bumps [pygit2](https://github.com/libgit2/pygit2) from 1.19.3 to 1.20.0.
- [Release notes](https://github.com/libgit2/pygit2/releases)
- [Changelog](https://github.com/libgit2/pygit2/blob/master/CHANGELOG.md)
- [Commits](libgit2/pygit2@v1.19.3...v1.20.0)

---
updated-dependencies:
- dependency-name: pygit2
  dependency-version: 1.20.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]