log-tree: replace include of revision.h with simple forward declaration

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/diff-tree.c

@@ -9,6 +9,7 @@
 #include "submodule.h"
 #include "read-cache-ll.h"
 #include "repository.h"
+#include "revision.h"
 #include "tree.h"
 
 static struct rev_info log_tree_opt;