### Summary
This PR fixes an issue where the git graph wouldn't refresh its state
correctly unless HEAD changed. Now repository emits events when the
branch list has changed, and invalidates the graph data cache when the
stash or branch list has changed.
I also renamed the event `Repository::BranchedChanged` to
`Repository::HeadChanged` as well.
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#53080
Release Notes:
- N/A or Added/Fixed/Improved ...
Anthony Eid
7748047051