Document git-diff-tree --always

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-diff-tree.txt

@@ -95,6 +95,10 @@ separated with a single space are given.
 	hunks disappear, the commit itself and the commit log
 	message is not shown, just like any other "empty diff" cases.
 
+--always::
+	Show the commit itself and the commit log message even
+	if the diff itself is empty.
+
 
 Limiting Output
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