Merge branch 'dz-improve-mr-versions' into 'master'

Fix duplicate master entries in the mr versions dropdown

## What does this MR do?

Fixes bug when "master" was duplicated per each mr version in the dropdown

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/22519, Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/22715

See merge request !6567

CHANGELOG

@@ -23,6 +23,7 @@ v 8.12.2 (unreleased)
   - Fix an issue with the "Commits" section of the cycle analytics summary. !6513
   - Fix errors importing project feature and milestone models using GitLab project import
   - Make JWT messages Docker-compatible
+  - Fix duplicate branch entry in the merge request version compare dropdown
 
 v 8.12.1
   - Fix a memory leak in HTML::Pipeline::SanitizationFilter::WHITELIST