fixup! reftable: write correct max_update_index to header

The original commit was missing some initializations. This lead to the
somewhat intuitive (and not reliably reproducible, until the trick was
found to use `sanitize=address,undefined`) symptom that t1400.249 and/or
t2400.171 failed with:

  Assertion failed: (ret != REFTABLE_API_ERROR), function
  reftable_be_transaction_finish, file reftable-backend.c, line 1648.

or

  Assertion failed: (ret != REFTABLE_API_ERROR), function
  write_transaction_table, file reftable-backend.c, line 1619.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: KarthikNayak

refs/reftable-backend.c

@@ -1020,6 +1020,7 @@ static int prepare_transaction_update(struct write_transaction_table_arg **out,
 		arg->updates_nr = 0;
 		arg->updates_alloc = 0;
 		arg->updates_expected = 0;
+		arg->max_index = 0;
 	}
 
 	arg->updates_expected++;
@@ -1634,6 +1635,8 @@ static int reftable_be_transaction_finish(struct ref_store *ref_store UNUSED,
 		tx_data->args->max_index = transaction->max_index;
 
 	for (size_t i = 0; i < tx_data->args_nr; i++) {
+		tx_data->args[i].max_index = transaction->max_index;
+
 		ret = reftable_addition_add(tx_data->args[i].addition,
 					    write_transaction_table, &tx_data->args[i]);
 		if (ret < 0)