compose: Print transaction when composing extensions

Makes it on par with the base compose stuff. The information about which
package is coming from which repo is really useful for debugging things.

src/app/rpmostree-compose-builtin-tree.cxx

@@ -1686,6 +1686,8 @@ rpmostree_compose_builtin_extensions (int argc, char **argv, RpmOstreeCommandInv
   if (!rpmostree_context_prepare (ctx, FALSE, cancellable, error))
     return FALSE;
 
+  rpmostree_print_transaction (rpmostree_context_get_dnf (ctx));
+
   if (!glnx_shutil_mkdir_p_at (AT_FDCWD, opt_extensions_output_dir, 0755, cancellable, error))
     return FALSE;