Improve formatting of fallback diff note

lib/gitlab/bitbucket_server_import/importer.rb

@@ -261,8 +261,13 @@ module Gitlab
 
       def create_fallback_diff_note(merge_request, comment, position)
         attributes = pull_request_comment_attributes(comment)
-        attributes[:note] = "*Comment on #{position.old_path}:#{position.old_line} -> #{position.new_path}:#{position.new_line}*\n\n" + attributes[:note]
+        note = "*Comment on "
 
+        note += " #{position.old_path}:#{position.old_line} -->" if position.old_line
+        note += " #{position.new_path}:#{position.new_line}" if position.new_line
+        new += "*\n\n#{comment.note}"
+
+        attributes[:note] = note
         merge_request.notes.create!(attributes)
       end