fixed incorrect reference to @repository.root_ref in _readme.html.haml. #22083

CHANGELOG

@@ -143,6 +143,7 @@ v 8.12.0 (unreleased)
   - Fix Gitlab::Popen.popen thread-safety issue
   - Add specs to removing project (Katarzyna Kobierska Ula Budziszewska)
   - Clean environment variables when running git hooks
+  - Fix non-master branch readme display in tree view
 
 v 8.11.6
   - Fix unnecessary horizontal scroll area in pipeline visualizations. !6005

app/views/projects/tree/_readme.html.haml

 %article.file-holder.readme-holder
   .file-title
     = blob_icon readme.mode, readme.name
-    = link_to namespace_project_blob_path(@project.namespace, @project, tree_join(@repository.root_ref, @path, readme.name)) do
+    = link_to namespace_project_blob_path(@project.namespace, @project, tree_join(@ref, @path, readme.name)) do
       %strong
         = readme.name
   .file-content.wiki