Fix `Rugged::Tree.diff` to allow the first given tree to be `nil`.
@@ -391,6 +391,8 @@ static VALUE rb_git_diff_tree_to_tree(VALUE self, VALUE rb_repo, VALUE rb_tree,
struct nogvl_diff_args args;
Data_Get_Struct(rb_repo, git_repository, repo);
+
+ if(RTEST(rb_tree))
Data_Get_Struct(rb_tree, git_tree, tree);
if(RTEST(rb_other_tree))
