Documentation: git-diff-tree --cc also omits empty commits

A misguided attempt to show logs at all time was inserted only to
the documentation of this flag.  Worse yet, it was not even implemented,
causing more confusion.  Drop it.

We might want to have an option to show --pretty even when there is no
diff output, but that is applicable to all forms of diff, not just --cc.

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