Use 'test_atexit' to run cleanup commands to stop the credentials
helper at the end of the test script or upon interrupt or failure, as
it is shorter, simpler, and more robust than registering such cleanup
commands in the trap on EXIT in the test scripts.
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>