This changes the Commit class to use new-style class, which has
been available since Python 2.2 (Dec 2001). This is a necessary
step in order to use __slots__[] declaration, so that we can
reduce the memory footprint in the next patch.
Signed-off-by: Michael Ellerman <michael@ellerman.id.au> Signed-off-by: Junio C Hamano <gitster@pobox.com>