diff.h: reduce unnecessary includes

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/tree-diff.c b/tree-diff.c
index 9ea2dd7..20bb15f 100644
--- a/tree-diff.c
+++ b/tree-diff.c
@@ -5,6 +5,7 @@
 #include "diff.h"
 #include "diffcore.h"
 #include "tree.h"
+#include "tree-walk.h"
 
 /*
  * Some mode bits are also used internally for computations.