Don't allow merge on top of pending merges without --force.

* bzrlib/tests/blackbox/test_merge.py:
(TestMergeForce.test_merge_force,
TestMergeForce.test_merge_with_uncommitted_changes,
TestMergeForce): Complete tests around --force including pending
merges.

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