git: Use repo snapshot to fetch picker branch list instead of spawning a job

[Anthony-Eid]

This should fix a delay where the thread_branch_picker and git_picker would take a while to load the branch list while other git jobs are being processed before hand.

I also added a subscription in both pickers to repository so they refresh their matches when the repo snapshot branch list changes.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• N/A