Bump git2 in /gc/mmtk in the cargo group across 1 directory

Bumps the cargo group with 1 update in the /gc/mmtk directory: [git2](https://github.com/rust-lang/git2-rs).


Updates `git2` from 0.20.2 to 0.20.4
- [Changelog](https://github.com/rust-lang/git2-rs/blob/git2-0.20.4/CHANGELOG.md)
- [Commits](rust-lang/git2-rs@git2-0.20.2...git2-0.20.4)

---
updated-dependencies:
- dependency-name: git2
  dependency-version: 0.20.4
  dependency-type: indirect
  dependency-group: cargo
...

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

Author: dependabot[bot]