object-store-ll.h: split this header out of object-store.h

The vast majority of files including object-store.h did not need dir.h
nor khash.h.  Split the header into two files, and let most just depend
upon object-store-ll.h, while letting the two callers that need it
depend on the full object-store.h.

After this patch:
    $ git grep -h include..object-store | sort | uniq -c
          2 #include "object-store.h"
        129 #include "object-store-ll.h"

Diff best viewed with `--color-moved`.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/replace-object.h b/replace-object.h
index 500482b..a964db1 100644
--- a/replace-object.h
+++ b/replace-object.h
@@ -3,7 +3,7 @@
 
 #include "oidmap.h"
 #include "repository.h"
-#include "object-store.h"
+#include "object-store-ll.h"
 
 /*
  * Do replace refs need to be checked this run?  This variable is