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/notes-merge.c b/notes-merge.c
index 4be11a0..0719478 100644
--- a/notes-merge.c
+++ b/notes-merge.c
@@ -5,7 +5,7 @@
 #include "refs.h"
 #include "object-file.h"
 #include "object-name.h"
-#include "object-store.h"
+#include "object-store-ll.h"
 #include "path.h"
 #include "repository.h"
 #include "diff.h"