Closes #55227

Updates the Git panel commit message editor to respect the configured
`buffer_font_size` when using buffer/editor typography. This keeps the
commit box visually consistent with other editor text while preserving
the existing UI-font sizing path for non-buffer panel editor styles.

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:

- Fixed the Git panel commit message editor ignoring `buffer_font_size`