Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] 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

Fixes hardcoded SHA lengths not using the `SHORT_SHA_LENGTH` constant.

Release Notes:

- Reduced short commit SHAs in blame and commit tooltips from 8 to 7 -
consistent with the graph and history views.