Fixes this single-frame glitch that sometimes occurs when the git blame
popover is rendered before the markdown has been parsed.


https://github.com/user-attachments/assets/d5a322c3-e46f-4597-a10f-a676da57daa7

## Self-Review Checklist

<!-- Check before requesting review: -->
- [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)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed a visual glitch where the git blame popover could briefly appear
empty while its markdown content was being parsed