Is Rugged::Commit.diff right?

I use walker find a commit but when i get it`s diff, i think the result is wrong.

rugged/lib/rugged/commit.rb

    def diff(*args)
      args.unshift(parents.first) if args.size == 1 && args.first.is_a?(Hash)
      self.tree.diff(*args)
    end

rugged/lib/rugged/tree.rb

    def diff(other = nil, options = nil)
      Tree.diff(repo, self, other, options)
    end

but
rugged/ext/rugged/rugged_tree.c

 *  call-seq:
 *    Tree.diff(repo, tree, diffable[, options]) -> diff
 *
 *  Returns a diff between the `tree` and the diffable object that was given.
 *  +diffable+ can either be a +Rugged::Commit+, a +Rugged::Tree+, a +Rugged::Index+,
 *  or +nil+.
 *
 *  The +tree+ object will be used as the "old file" side of the diff, while the
 *  parent tree or the +diffable+ object will be used for the "new file" side.

so all seams right, but as the comment say The +tree+ object will be used as the "old file" side of the diff, while the parent tree or the +diffable+ object will be used for the "new file" side, is parent used as new side right?

Trace the call flow in rugged/lib/rugged/commit.rb and rugged/lib/rugged/tree.rb into the Tree.diff implementation in rugged/ext/rugged/rugged_tree.c. Reproduce the reported walker commit diff, then compare the observed old and new sides with the C documentation and clarify whether the parent is used as the new side.