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 --git a/range-diff.c b/range-diff.c
index 6a704e6..2e86063 100644
--- a/range-diff.c
+++ b/range-diff.c
@@ -13,6 +13,7 @@
 #include "commit.h"
 #include "pager.h"
 #include "pretty.h"
+#include "repository.h"
 #include "userdiff.h"
 #include "apply.h"
 #include "revision.h"