make the Ruby version look more like the C version did

Author: tenderlove

lib/rugged/index.rb

@@ -109,12 +109,10 @@ def diff(*args)
       else
         if other.is_a? ::Rugged::Commit
           diff_tree_to_index other.tree, options
+        elsif other.is_a? ::Rugged::Tree
+          diff_tree_to_index other, options
         else
-          if other.is_a? ::Rugged::Tree
-            diff_tree_to_index other, options
-          else
-            raise TypeError, "A Rugged::Commit or Rugged::Tree instance is required"
-          end
+          raise TypeError, "A Rugged::Commit or Rugged::Tree instance is required"
         end
       end
     end