object-store: move struct packed_git into "packfile.h"

The "object-store.h" header contains the definition of `struct
packed_git`. As this structure hosts all kind of information about a
specific packfile it is arguably a bit out of place in a generic place
like "object-store.h".

Move the structure as well as `pack_map_entry_cmp()` into "packfile.h".

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-objects.h

@@ -4,6 +4,7 @@
 #include "object-store.h"
 #include "thread-utils.h"
 #include "pack.h"
+#include "packfile.h"
 
 struct repository;