Don't allow plus and minus in diff copy paste data

app/assets/stylesheets/pages/diff.scss

@@ -62,6 +62,12 @@
       .line_holder td {
         line-height: $code_line_height;
         font-size: $code_font_size;
+        &.noteable_line.old:before {
+          content: '-';
+        }
+        &.noteable_line.new:before {
+          content: '+';
+        }
       }
     }

app/helpers/diff_helper.rb

@@ -44,6 +44,7 @@ module DiffHelper
     if line.blank?
       "  ".html_safe
     else
+      line[0] = ''
       line
     end
   end