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>

Documentation/git-diff-tree.txt

@@ -93,7 +93,7 @@ separated with a single space are given.
 	parent, or show the same change from all but one parent
 	for an Octopus merge.  When this optimization makes all
 	hunks disappear, the commit itself and the commit log
-	message is not shown, unless '-m' is specified.
+	message is not shown, just like any other "empty diff" cases.
 
 
 Limiting Output