Use raw_diffs to calculate MR files changed

We don't need to create the intermediate FileCollection object with its
associated highlighting in order to count how many files changed. Both
the compare object and the MR diff object have a raw_diffs method that
is perfectly fine for this case.

app/models/merge_request.rb

@@ -198,7 +198,9 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def diff_size
-    diffs(diff_options).size
+    opts = diff_options || {}
+
+    raw_diffs(opts).size
   end
 
   def diff_base_commit