Don't allow remove-tree to succeed with pending merges unless
--force is used.

* bzrlib/tests/blackbox/test_remove_tree.py:
(TestRemoveTree.test_remove_tree_pending_merges,
TestRemoveTree.test_remove_tree_pending_merges_force): Test with
and without --force and pending merges.

* bzrlib/builtins.py:
(cmd_remove_tree.run): Check pending merges too.