render: add a count to the diff headings

CHANGELOG.md

@@ -8,6 +8,8 @@ This is a changelog of the `dix` repository. It follows the
 
 ### Added
 
+- The `CHANGED`, `ADDED` and `REMOVED` headings now show how many packages
+  they contain, e.g. `CHANGED (12)`.
 - Added the `dix last [N]` subcommand, which shows the changes leading up to
   the current profile generation, `dix last --all` for every generation, and
   `dix last --from A --to B` for the generations numbered A to B.

crates/dix/src/render.rs

@@ -98,8 +98,13 @@ fn render_diffs(
         StatusGroup::Removed => "REMOVED",
       }
       .bold();
+      let count = diffs
+        .iter()
+        .filter(|other| status_group(other.status) == group)
+        .count();
+      let count = format!("({count})");
 
-      writeln!(writer, "{header}")?;
+      writeln!(writer, "{header} {count}", count = count.dim())?;
       last_status_group = Some(group);
     }