git: Further extract gitlib2 dependencies (#58280)

Extraction done from #53453

I removed the default Oid implementation we had and added support back
for SHA264 back as well. I also removed the hex dependency and just
added some of those functions we needed in house so we can avoid
building yet another dependency

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A