Ensure link for README blobs includes the full path

Closes #13254

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