diff.h: remove unnecessary include of oidset.h

This also made it clear that several .c files depended upon various
things that oidset included, but had omitted the direct #include for
those headers.  Add those now.

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

tree-diff.c

@@ -4,6 +4,7 @@
 #include "git-compat-util.h"
 #include "diff.h"
 #include "diffcore.h"
+#include "hash.h"
 #include "tree.h"
 #include "tree-walk.h"