Make left-right automatic.

Make left-right automatic.

When using symmetric differences, I think the user almost always
would want to know which side of the symmetry each commit came
from.  So this removes --left-right option from the command
line, and turns it on automatically when a symmetric difference
is used ("git log --merge" counts as a symmetric difference
between HEAD and MERGE_HEAD).

Just in case, a new option --no-left-right is provided to defeat
this, but I do not know if it would be useful.

Signed-off-by: Junio C Hamano <junkio@cox.net>