Display renamed files in diff views

Show both the old and new filenames when viewing the diff for a renamed
file.

app/views/projects/diffs/_file.html.haml

@@ -9,6 +9,9 @@
       .diff-btn-group
         - if @commit.parent_ids.present?
           = view_file_btn(@commit.parent_id, diff_file, project)
+    - else
+      - if diff_file.renamed_file
+        %span= "#{diff_file.old_path} renamed to #{diff_file.new_path}"
       - else
         %span= diff_file.new_path
       - if diff_file.mode_changed?