diff-tree --always flag

diff-tree --always flag

It _might_ make sense for certain users like gitk and gitview if
we had a single tool that gives --pretty and its diff even if
the diff is empty.  Having said that, the flag --cc -m is too
specific.  If some uses want to see the commit log even for an
empty diff, that flag should not be something only --cc honors.

Here's an "--always" flag that does that.

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