Re-fix the priority order since there is a known valid case.
* bzrlib/tests/test_osutils.py:
(TestTerminalWidth.test_falls_back_to_COLUMNS): Ensures COLUMNS is
taken into account if BZR_COLUMNS is not set.
* bzrlib/osutils.py:
(terminal_width): COLUMNS takes priority on termios.TIOCGWINSZ as
shown by tests under emacs (on no counter examples are known).