move tree handling to Ruby

Author: tenderlove

ext/rugged/rugged_tree.c

@@ -489,12 +489,7 @@ static VALUE rb_git_tree_diff_(VALUE self, VALUE rb_repo, VALUE rb_self, VALUE r
 		Data_Get_Struct(rb_self, git_tree, tree);
 	}
 
-		if (rb_obj_is_kind_of(rb_other, rb_cRuggedTree)) {
-			git_tree *other_tree;
-
-			Data_Get_Struct(rb_other, git_tree, other_tree);
-			error = git_diff_tree_to_tree(&diff, repo, tree, other_tree, &opts);
-		} else if (rb_obj_is_kind_of(rb_other, rb_cRuggedIndex)) {
+		if (rb_obj_is_kind_of(rb_other, rb_cRuggedIndex)) {
 			git_index *index;
 			Data_Get_Struct(rb_other, git_index, index);
 			error = git_diff_tree_to_index(&diff, repo, tree, index, &opts);

lib/rugged/tree.rb

@@ -19,6 +19,8 @@ def self.diff(repo, _self, other = nil, options = {})
         case other
         when Rugged::Commit
           diff_tree_to_tree repo, _self, other.tree, options
+        when Rugged::Tree
+          diff_tree_to_tree repo, _self, other, options
         else
           _diff(repo, _self, other, options)
         end