[api] Use superseding.last instead of index

We want a comma except for the last record, because of that I think that
it is more readable to use `superseding.last` than using an index and
comparing it with `superseding.length - 1`. This way we also don't need
to use `each_with_index`, as we don't really care of what index every
record have.

This may be a matter of taste, but I find it more elegant.

src/api/app/views/webui/request/_superseded_by_message.html.haml

@@ -3,9 +3,9 @@
     .ui-state-info.ui-corner-all.ui-widget-shadow.padding-10pt#request-supersedes-hint
       %span.ui-icon.ui-icon-info
         This request supersedes:
-        - superseding.each_with_index do |bs_request, index|
+        - superseding.each do |bs_request|
           = link_to "request #{bs_request.number}", request_show_path(number: bs_request.number)
-          = surround '(', ")#{', ' if index < superseding.length - 1 }" do
+          = surround '(', ")#{', ' unless bs_request == superseding.last }" do
             = link_to('Show diff', request_show_path(number: bs_request.superseded_by, diff_to_superseded: bs_request.number))
 - if superseded_by.present?
   .grid_16.alpha.omega.box-invisible