Draft: Re-add "merged" badge to branches without changes to default branch

What does this MR do and why?

Contributes to #414845

Problem

merged badge was removed as a part of branch list refactoring: !119637 (merged).

The naming merged was confusing some users: #227112 (comment 659657416)

But other group of users relied on it for their workflows: #414845 (comment 1476293037)

Solution

Restore merged badge

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.