git: Add file compare actions for git refs

[nguyenphutrong]

Screenshot.2026-05-12.at.12.00.46.mp4

Summary:

• Added Git sidebar/editor actions to compare the selected file with a branch or commit.
• Reused the ProjectDiff-style viewer so file compares show base/current context and support existing unified/split controls.
• Added a Git submenu in the project panel context menu for file history and compare actions.

Tests:

• cargo fmt --check
• git diff --check
• rustup run 1.95.0 cargo check -p git_ui
• rustup run 1.95.0 cargo check -p project
• rustup run 1.95.0 cargo check -p project_panel
• cargo test -p git_ui
• cargo test -p project_panel
• ./script/clippy -p git_ui -p project_panel -p git

Suggested .rules additions:

• In git_ui tests that instantiate ProjectDiff directly, call crate::init(cx) after editor::init(cx) so editor add-ons such as ConflictAddon are registered like runtime.

Release Notes:

• Added Git actions to compare a file with a selected branch or commit.