repo: Add ostree_repo_get_dfd()

I plan to use this in rpm-ostree at least for two reasons:
 - To find the mtime on the repo
 - To use the tmp/ directory to stage content (but we should eventually
   add a better API)

src/libostree/ostree-repo.h

@@ -82,6 +82,9 @@ gboolean      ostree_repo_create (OstreeRepo     *self,
 _OSTREE_PUBLIC
 GFile *       ostree_repo_get_path (OstreeRepo  *self);
 
+_OSTREE_PUBLIC
+int           ostree_repo_get_dfd (OstreeRepo  *self);
+
 _OSTREE_PUBLIC
 OstreeRepoMode ostree_repo_get_mode (OstreeRepo  *self);