fix: gate Changes component on git feature flag

When bat is built without the `git` feature, `StyleComponent::Changes`
is not available. Add `#[cfg(feature = "git")]` guard so `--diff --plain`
works in both configurations.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/bin/bat/app.rs

@@ -596,6 +596,7 @@ impl App {
             if self.matches.try_contains_id("diff").unwrap_or_default()
                 && self.matches.get_flag("diff")
             {
+                #[cfg(feature = "git")]
                 components.insert(StyleComponent::Changes);
                 components.insert(StyleComponent::Snip);
             }