t4018: drop "debugging" cat from hunk-header tests

We run a series of hunk-header tests in a loop, and each one does this: test_when_finished 'cat actual' && # for debugging only This is pretty pointless. When the test succeeds, we waste time running a useless cat process. If you're debugging a failure with "-i", then we won't run the when-finished part at all. So it helps only if you're running with something like "--verbose-log". Since we expect the tests to succeed most of the time, a better way to do this would be a helper that checks the output and dumps "actual" only when it fails. But it's probably not even worth the effort, as anyone debugging a failure could just run with "-i" and investigate the "actual" file themselves. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>