Fixed as per Aarons' comment.

* bzrlib/tests/blackbox/test_log.py:
(TestLog): Add more tests for 'delta_format'.

* bzrlib/log.py:
(LogFormatter): Document the 'delta_format' attribute.
(LogFormatter.__init__): Initialize 'delta_format' respecting
bzckward compatibility.
(LongLogFormatter.log_revision): Takes 'delta_format' into account.
(ShortLogFormatter.log_revision): Takes 'delta_format' into account.

* bzrlib/builtins.py:
(cmd_log.run): Use get_verbosity_level() to distinguish between -v
and -vv.