submodule--helper: fix trivial get_default_remote_submodule() leak

Fix a leak in code added in 1012a5c (submodule--helper
run-update-procedure: learn --remote, 2022-03-04), we need to free()
the xstrdup()'d string. This gets e.g. t/t7419-submodule-set-branch.sh
closer to passing under SANITIZE=leak.

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Reviewed-by: Glen Choo <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

builtin/submodule--helper.c

@@ -2448,6 +2448,8 @@ static int update_submodule(struct update_data *update_data)
 			return code;
 		remote_ref = xstrfmt("refs/remotes/%s/%s", remote_name, branch);
 
+		free(remote_name);
+
 		if (!update_data->nofetch) {
 			if (fetch_in_submodule(update_data->sm_path, update_data->depth,
 					      0, NULL))