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/diff-lib.c b/diff-lib.c
index f3aa7f9..cad1809 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -7,6 +7,7 @@
 #include "diff.h"
 #include "diffcore.h"
 #include "gettext.h"
+#include "hash.h"
 #include "hex.h"
 #include "object-name.h"
 #include "read-cache.h"
@@ -14,6 +15,7 @@
 #include "cache-tree.h"
 #include "unpack-trees.h"
 #include "refs.h"
+#include "repository.h"
 #include "submodule.h"
 #include "symlinks.h"
 #include "trace.h"