merge: teach -Xours/-Xtheirs to binary ll-merge driver

merge: teach -Xours/-Xtheirs to binary ll-merge driver

The (discouraged) -Xours/-Xtheirs modes of merge are supposed to
give a quick and dirty way to come up with a random mixture of
cleanly merged parts and punted conflict resolution to take contents
from one side in conflicting parts.  These options however were only
passed down to the low level merge driver for text.

Teach the built-in binary merge driver to notice them as well.

Signed-off-by: Junio C Hamano <gitster@pobox.com>