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/http-push.c b/http-push.c
index 3f18498..866ba24 100644
--- a/http-push.c
+++ b/http-push.c
@@ -16,6 +16,7 @@
 #include "sigchain.h"
 #include "strvec.h"
 #include "tree.h"
+#include "tree-walk.h"
 #include "packfile.h"
 #include "object-store.h"
 #include "commit-reach.h"