Add missing includes and forward declarations

I looped over the toplevel header files, creating a temporary two-line C
program for each consisting of
  #include "git-compat-util.h"
  #include $HEADER
This patch is the result of manually fixing errors in compiling those
tiny programs.

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

object-store.h

@@ -1,6 +1,7 @@
 #ifndef OBJECT_STORE_H
 #define OBJECT_STORE_H
 
+#include "cache.h"
 #include "oidmap.h"
 #include "list.h"
 #include "sha1-array.h"