builtin/fetch: fix leaking transaction with --atomic

With the `--atomic` flag, we use a single ref transaction to commit all
ref updates in git-fetch(1). The lifetime of transactions is somewhat
weird: while `ref_transaction_abort()` will free the transaction, a call
to `ref_transaction_commit()` won't. We thus have to manually free the
transaction in the successful case.

Adapt the code to free the transaction in the exit path to plug the
resulting memory leak. As `ref_transaction_abort()` already freed the
transaction for us, we have to unset the transaction when we hit that
code path to not cause a double free.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5574-fetch-output.sh

@@ -5,6 +5,7 @@ test_description='git fetch output format'
 GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
 export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 test_expect_success 'fetch with invalid output format configuration' '