Use tree.has_changes() where appropriate (the test suite caught a
bug in has_changes() (not filtering out the root) in an impressive
number of tests)

* bzrlib/send.py:
(send): Use tree.has_changes() instead of tree.changes_from().

* bzrlib/reconfigure.py:
(Reconfigure._check): Use tree.has_changes() instead of
tree.changes_from().

* bzrlib/merge.py:
(Merger.ensure_revision_trees, Merger.compare_basis): Use
tree.has_changes() instead of tree.changes_from().

* bzrlib/builtins.py:
(cmd_remove_tree.run, cmd_push.run, cmd_merge.run): Use
tree.has_changes() instead of tree.changes_from().