core: Add malloc-free API for objects, use *at functions for storing

This is more efficient; we avoid malloc of a number of pathname +
GFile objects, plus the kernel doesn't have to traverse the repo path
again.

src/libostree/ostree-repo-private.h

@@ -34,6 +34,7 @@ struct OstreeRepo {
   GFile *local_heads_dir;
   GFile *remote_heads_dir;
   GFile *objects_dir;
+  int objects_dir_fd;
   GFile *uncompressed_objects_dir;
   GFile *remote_cache_dir;
   GFile *config_file;