git: Move diff hunk operations onto buffer diffs (#63556)

# Objective

Keep Git-specific diff-base knowledge in `GitStore`, where Git
operations are defined.

Related to FR-135.

## Solution

Give each `BufferDiff` its supported hunk operations. Keep editor hooks
limited to rendering custom controls.

## 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 adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

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Release Notes:

- N/A
229450844dBen Kunkle committed on 9/1/2026, 3:52:33 PM· committed by GitHubparentc4bc69e
21 files changedLine totals unavailable