This sends `branch_list` in live `UpdateRepository` messages so
downstream repository snapshots stay in sync and the remote branch
picker can populate correctly. Collab DB replay for join/reconnect will
follow in a separate change.

This fixes a bug where the branch picker wouldn't show any branches in
remote environments

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

Closes #54530

Release Notes:

- git: Fix remote branch picker not showing any branches