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>

diff.h

@@ -4,10 +4,12 @@
 #ifndef DIFF_H
 #define DIFF_H
 
+#include "hash-ll.h"
 #include "pathspec.h"
-#include "oidset.h"
 #include "strbuf.h"
 
+struct oidset;
+
 /**
  * The diff API is for programs that compare two sets of files (e.g. two trees,
  * one tree and the index) and present the found difference in various ways.