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 #55880 

This improves the work on #56569 by allowing the user to choose the base
branch to compare with directly, instead of having to open the default
branch diff and only after, picking the new base branch.

Additionally, this changes the existing item finder to also match by
base (oid). This allows to have multiple branch diff open with different
base branches.

Demo: 



https://github.com/user-attachments/assets/8abdc08f-6181-4a74-a50b-e5f2b891663e



Release Notes:

- Added new `git: compare with branch` action to directly compare the
current branch with an arbitrary branch.