Re-add missing merge date tooltip to Vue MR list page

What does this MR do and why?

For part of #504362 (closed)

Adds a wee tooltip to the "Merged" badge on the MR list.

This tooltip is missing since the page was refactored from HAML to Vue.

The code does this by adding the tooltip to the generic issuable_item, but the value is only filled with some text (the timestamp) if the issuable has a mergedAt value, which should only exist for merged-state issuables (aka merged Merge Requests).

MR acceptance checklist

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Screenshots or screen recordings

The before image is showing no tooltip (or any hover effect at all).

Before	After

How to set up and validate locally

1. Have a merged MR
2. Go to the Merge Requests page
3. Click the "Merged" tab
4. Hover over the "Merged" badge

app/assets/javascripts/merge_requests/list/queries/merge_request.fragment.graphql

@@ -9,6 +9,7 @@ fragment MergeRequestFragment on MergeRequest {
   state
   title
   updatedAt
+  mergedAt
   upvotes
   userDiscussionsCount @include(if: $isSignedIn)
   webUrl