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

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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

changed milestone to %17.7

added Deliverable devopscreate featureenhancement frontend groupcode review sectiondev twtriaged typefeature workflowin dev labels

assigned to @thomasrandolph

added 1 commit

• d7f2a6ff - Add missing tooltip to "Merged" status MRs in issuable lists

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	2 Warnings
	featureaddition and featureenhancement merge requests normally have a documentation change. Consider adding a documentation update or confirming the documentation plan with the Technical Writer counterpart. For more information, see: The Handbook page on merge request types. The definition of done documentation.
	This merge request changed undocumented Vue components in vue_shared/. Please consider creating Stories for these components: app/assets/javascripts/vue_shared/issuable/list/components/issuable_item.vue

Reviewer roulette

Category	Reviewer	Maintainer
frontend	@lwanko (UTC+1, 8 hours ahead of author)	@psjakubowska (UTC+1, 8 hours ahead of author)

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Generated by Danger

marked this merge request as ready

changed title from Draft: Resolve "Address feedback from rolling out vue refactor of MR List" to Re-add missing merge date tooltip to Vue MR list page

changed the description

added 1 commit

• 3af397a3 - Add missing tooltip to "Merged" status MRs in issuable lists

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added 1 commit

• c6331f3a - Add missing tooltip to "Merged" status MRs in issuable lists

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Generated by gitlab_quality-test_tooling.

---

Slow tests detected in this merge request. These slow tests might be related to this merge request's changes.

Click to expand

Job	File	Name	Duration	Expected duration
#8629677504	spec/features/discussion_comments/merge_request_spec.rb#L20	Thread Comments Merge Request behaves like thread comments for issue, epic and merge request clicking "Comment" will post a comment	50.34 s	< 50.13 s

added rspec:slow test detected label

added 325 commits

• c6331f3a...0b898127 - 324 commits from branch master
• 09689cad - Add missing tooltip to "Merged" status MRs in issuable lists

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changed the description

@lwanko can you start the review here?

requested review from @lwanko

added 1 commit

• 7fd0ab01 - Add missing tooltip to "Merged" status MRs in issuable lists

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