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>

pack-write.c

@@ -11,6 +11,7 @@
 #include "pack-objects.h"
 #include "pack-revindex.h"
 #include "path.h"
+#include "strbuf.h"
 #include "wrapper.h"
 
 void reset_pack_idx_option(struct pack_idx_option *opts)