core: Ensure we delete duplicate files in a transaction

If multiple files have the same hash, we need to ensure we're not
overwriting other tempfiles in the same transaction.  Instead
just delete them, since we know they're in the repo.

src/libostree/ostree-core.c

@@ -887,7 +887,7 @@ ostree_create_temp_file_from_input (GFile            *dir,
       g_free (possible_name);
       possible_name = subst_xxxxxx (tmp_name->str);
       g_clear_object (&possible_file);
-      possible_file = ot_gfile_new_for_path (possible_name);
+      possible_file = g_file_get_child (dir, possible_name);
       
       if (!ostree_create_file_from_input (possible_file, finfo, xattrs, input,
                                           objtype,