### Summary

Added a branch diff base branch picker that shows the selected diff base
and prioritizes branches on the active branch’s remote. This also
generalizes the branch picker’s checkout vs. select only behavior so its
UI/search/rendering can be reused by a future git graph picker, and
makes branch diff drop stale in progress base update tasks when the base
or repository changes for faster response.

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  #54050

Release Notes:

- git: Allow choosing branch diff base in branch diff view

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