Add visibility for MR links for non-default branches
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=475150) </details> <!--IssueSummary end--> <!-- This template is a great use for issues that are feature::additions or technical tasks for larger issues.--> ### Proposal When accessing **Code > Branches** to [view all branches](https://docs.gitlab.com/ee/user/project/repository/branches/#view-all-branches), it shows you the links to the Merge Requests on that branch. As of writing this will only show MRs that are pointed to the default branch (e.g. main). We should allow users to see MRs for all target branches or give them an option to filter which target branch to show. ![missing_mr_non_default](/uploads/25a271ed5ee9c44d028ad7ce978834cb/missing_mr_non_default.png) <!-- Consider adding related issues and epics to this issue. You can also reference the Feature Proposal Template (https://gitlab.com/gitlab-org/gitlab/-/blob/master/.gitlab/issue_templates/Feature%20proposal%20-%20detailed.md) for additional details to consider adding to this issue. Additionally, as a data oriented organization, when your feature exits planning breakdown, consider adding the `What does success look like, and how can we measure that?` section. --> <!-- Label reminders Use the following resources to find the appropriate labels: - Use only one tier label choosing the lowest tier this is intended for - https://gitlab.com/gitlab-org/gitlab/-/labels - https://about.gitlab.com/handbook/product/categories/features/ -->
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