Fixes git graph lane selection when multiple active lanes point to the
same parent commit, matching Git’s `--graph` behavior by keeping the
commit on the leftmost incoming lane instead of the first discovered
lane.

This should make it much easier to track long lived branches 

### Before
<img width="318" height="611" alt="image"
src="https://github.com/user-attachments/assets/b6799992-1a96-4c35-ad38-dd0c7d2ae0e6"
/>

### After
<img width="180" height="596" alt="Screenshot 2026-05-05 at 4 07 28 PM"
src="https://github.com/user-attachments/assets/a8727d59-c423-49c3-aad2-0097f23d9251"
/>



 
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

---------

Co-authored-by: Remco Smits <djsmits12@gmail.com>