Fix diff header with submodule change

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

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

@@ -10,8 +10,9 @@
         - if @commit.parent_ids.present?
           = view_file_btn(@commit.parent_id, diff_file, project)
     - elsif diff_file.diff.submodule?
-      - submodule_item = project.repository.blob_at(@commit.id, diff_file.file_path)
-      = submodule_link(submodule_item, @commit.id, project.repository)
+      %span
+        - submodule_item = project.repository.blob_at(@commit.id, diff_file.file_path)
+        = submodule_link(submodule_item, @commit.id, project.repository)
     - else
       %span
         - if diff_file.renamed_file