remove detect_enoding. detect_encoding means force_encoding to the file.data's encoding, not encoding to utf8. will cause encoding problem.

app/views/refs/_tree.html.haml

@@ -42,9 +42,9 @@
         .readme
           - if content.name =~ /\.(md|markdown)$/i
             = preserve do
-              = markdown(content.data.detect_encoding!)
+              = markdown(content.data)
           - else
-            = simple_format(content.data.detect_encoding!)
+            = simple_format(content.data)
 
 - if params[:path]
   - history_path = tree_file_project_ref_path(@project, @ref, params[:path])

app/views/refs/_tree_file.html.haml

@@ -13,7 +13,7 @@
       #tree-readme-holder
         .readme
           = preserve do
-            = markdown(file.data.detect_encoding!)
+            = markdown(file.data)
     - else
       .view_file_content
         - unless file.empty?