Add --disable-fsync option to pull-local, and API to repo

This will be used by guestmount - it's WAY faster.  We only take disks
as a unit, so it's safe.  If the process fails halfway through, we
just start over from scratch the next time anyways.

src/libostree/ostree-repo-private.h

@@ -57,6 +57,7 @@ struct OstreeRepo {
 
   gboolean inited;
   gboolean in_transaction;
+  gboolean disable_fsync;
   GHashTable *loose_object_devino_hash;
   GHashTable *updated_uncompressed_dirs;
   GHashTable *object_sizes;