When a category page is edited and pages are added to the category, the "group results" option on the watchlist page results in an unexpected and not useful diff link.

To reproduce:

1. In Preferences>Watchlist, ensure "Hide categorization of pages" is not selected.
2. Edit a watched category page, then add one or more pages to the category.
3. In watchlist, turn on the "category changes" filter, and check "group results by page".
4. Click "diff" on the entry corresponding to the edited category.

Actual result:
One watchlist entry is presented which merges both the edits to the category itself, and the addition of that category to other pages. The "diff" link uses the page ID of the last page added to the category, rather than that of the category itself. This alarmingly makes it appear that the contents of the category page have been completely replaced with the contents of the last-added member page.

Example: this diff was given in Watchlist after the category was edited and members were added.

Expected result:
Edits to a category page itself should be grouped in one watchlist entry, with a diff, and edits that add members to a watched category should be grouped in a separate entry, without a diff, with the summary "X pages added to category".