Use __cmp__ instead of __eq__ for Tree comparison.

Use __cmp__ instead of __eq__ for Tree comparison.

This ensures that __ne__ will also work, and makes it easier to
subclass Tree.  For example, in the previous implementation you could
have
  >>> commA == commB
  False
  >>> cmp(commA, commB)
  0
if the comments had different ids, but equivalent content.

At the user-interface level, this removes some false "modified
comments" from `be diff`.