t5505: modernize and simplify hard-to-digest test

This test uses a subshell within a subshell but is formatted in such a way as to suggests that the inner subshell is a sibling rather than a child, which makes it difficult to digest the test's structure and intent. Worse, the inner subshell performs cleanup of actions from earlier in the test, however, a failure between the initial actions and the cleanup will prevent the cleanup from taking place. Fix these problems by modernizing and simplifying the test and by using test_when_finished() for the cleanup action. Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>