core: Use at-relative lookup for locating objects too

Add new internal API to both fstatat() and write a pathname for the
given object.  Use it in commit, and also wrapped in the old
GFile-based API.

This is more efficient.

src/libostree/ostree-repo-private.h

@@ -76,6 +76,15 @@ _ostree_repo_find_object (OstreeRepo           *self,
                           GCancellable         *cancellable,
                           GError             **error);
 
+gboolean
+_ostree_repo_has_loose_object (OstreeRepo           *self,
+                               const char           *checksum,
+                               OstreeObjectType      objtype,
+                               gboolean             *out_is_stored,
+                               char                 *loose_path_buf,
+                               GCancellable         *cancellable,
+                               GError             **error);
+
 gboolean
 _ostree_repo_get_loose_object_dirs (OstreeRepo       *self,
                                     GPtrArray       **out_object_dirs,