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

---

Release Notes:

- N/A

crates/action_log/src/action_log.rs

@@ -160,13 +160,9 @@ impl ActionLog {
                 let language = buffer.read(cx).language().cloned();
                 let language_registry = buffer.read(cx).language_registry();
                 let diff = cx.new(|cx| {
-                    BufferDiff::new(
-                        &text_snapshot,
-                        language,
-                        language_registry,
-                        buffer_diff::DiffBaseKind::Custom,
-                        cx,
-                    )
+                    let mut diff = BufferDiff::new(&text_snapshot, language, language_registry, cx);
+                    diff.set_operations(Arc::new(buffer_diff::RestoreDiffOperations));
+                    diff
                 });
                 let (diff_update_tx, diff_update_rx) = mpsc::unbounded();
                 let diff_base;