feat: preserve change markers when combining --diff with --plain

When --plain is set (via CLI or config file), --diff output loses all
visual markers, making it indistinguishable from plain text. The diff
line filtering still works, but without Changes markers or Snip
separators the output is not useful.

Automatically include Changes and Snip style components when --diff is
active alongside --plain. This preserves the --plain intent (no grid,
no header, no line numbers) while keeping diff output readable.

Closes #3630

CHANGELOG.md

@@ -5,6 +5,7 @@
 
 ## Features
 
+- Preserve `--diff` change markers and snip separators when `--plain` is set, see #3630 (@mvanhorn)
 - Added support for `hidden_file_extensions` from `.sublime-syntax` files, see #3613 (@Matei02355)
 - Add word wrapping mode via `--wrap=word`, see #3597 (@veeceey)
 - Implement `--unbuffered` mode for streaming input, allowing partial lines to display immediately (e.g. `tail -f | bat -u`). Closes #3555, see #3583 (@mainnebula)

src/bin/bat/app.rs

@@ -590,7 +590,16 @@ impl App {
 
         // Plain if `--plain` is specified at least once.
         if self.matches.get_count("plain") > 0 {
-            return Some(StyleComponents(HashSet::from([StyleComponent::Plain])));
+            let mut components = HashSet::from([StyleComponent::Plain]);
+            // When --diff is active, preserve change markers and snip separators
+            // so that diff output remains visually useful.
+            if self.matches.try_contains_id("diff").unwrap_or_default()
+                && self.matches.get_flag("diff")
+            {
+                components.insert(StyleComponent::Changes);
+                components.insert(StyleComponent::Snip);
+            }
+            return Some(StyleComponents(components));
         }
 
         // Default behavior.