Bump nodegit from 0.28.0-alpha.20 to 0.28.0-alpha.21 (#159)

Bumps [nodegit](https://github.com/nodegit/nodegit) from 0.28.0-alpha.20 to 0.28.0-alpha.21.
- [Release notes](https://github.com/nodegit/nodegit/releases)
- [Changelog](https://github.com/nodegit/nodegit/blob/master/CHANGELOG.md)
- [Commits](nodegit/nodegit@v0.28.0-alpha.20...v0.28.0-alpha.21)

---
updated-dependencies:
- dependency-name: nodegit
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]