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>

merge.c

@@ -2,10 +2,12 @@
 #include "diff.h"
 #include "diffcore.h"
 #include "gettext.h"
+#include "hash.h"
 #include "hex.h"
 #include "lockfile.h"
 #include "merge.h"
 #include "commit.h"
+#include "repository.h"
 #include "run-command.h"
 #include "resolve-undo.h"
 #include "tree.h"