Don't rebuild diverging commit count caches

When calling Repository#build_cache we _don't_ want to build the
diverging commit count cache as doing so can be _very_ slow for
repositories with lots of branches. Because these caches are built
whenever needed (= when actually viewing the list of branches in the web
UI) we can safely remove this code from
Repository#build_cache.

app/models/repository.rb

@@ -227,12 +227,6 @@ class Repository
         send(key)
       end
     end
-
-    branches.each do |branch|
-      unless cache.exist?(:"diverging_commit_counts_#{branch.name}")
-        send(:diverging_commit_counts, branch)
-      end
-    end
   end
 
   def expire_tags_cache