git_ui: Add staged and unstaged change views

[shenghsi]

Summary

• add separate staged and unstaged project-diff views
• keep staged diffs refreshed from HEAD to the index while preserving selections
• expose actions from the Git panel and entry context menus
• cover partial staging, deletion, rename, refresh, reuse, and repository switching

Upstream adaptation

Selectively reimplements the fork-compatible behavior from zed-industries/zed#46541 without importing its collaboration and broad buffer-diff refactors.

Validation

• cargo test -p git_ui --lib -- --test-threads=1 (119 passed)
• cargo test -p git_ui project_diff::tests::test_staged -- --test-threads=1 (4 passed)
• cargo fmt --all -- --check
• ./script/clippy
• fresh /tmp/Flint-Local.app bundle verified with codesign

Release Notes:

• Added separate staged and unstaged change views.