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/diff.c b/diff.c
index d02fbf5..6fb2946 100644
--- a/diff.c
+++ b/diff.c
@@ -20,7 +20,7 @@
 #include "attr.h"
 #include "run-command.h"
 #include "utf8.h"
-#include "object-store.h"
+#include "object-store-ll.h"
 #include "userdiff.h"
 #include "submodule-config.h"
 #include "submodule.h"