Fix lastgenre "count" issue

When original genres were kept (keep_existing option), the final genre
count was "off". The reason was that reducing genres to that count is
handled in _resolve_genre which wasn't run.

- This fixes it by ensuring a run of _resolve_genre in
  _combine_and_label_genres.
- There is a small caveat though: New genres have been run through
  _resolve_genres already. When they are combined with the old ones,
  they run through it again. Let's take this into account for now and
  hope performance doesn't suffer too much.

beetsplug/lastgenre/__init__.py

@@ -370,8 +370,11 @@ class LastGenrePlugin(plugins.BeetsPlugin):
         if new_genres and keep_genres:
             split_genres = new_genres.split(separator)
             combined_genres = deduplicate(keep_genres + split_genres)
-            return separator.join(combined_genres), f"keep + {log_label}"
+            # If we combine old and new, run through _resolve_genres (again).
+            resolved_genres = self._resolve_genres(combined_genres)
+            return resolved_genres, f"keep + {log_label}"
         if new_genres:
+            # New genres ran through _resolve_genres already.
             return new_genres, log_label
         return None, log_label