t5319: fix bogus cat-file argument

t5319: fix bogus cat-file argument

There's no such argument as "--unsorted"; it's spelled "--unordered".
But our test failed to notice that cat-file didn't run at all because:

  1. It lost the exit code of git on the left-hand side of a pipe.

  2. It was comparing two runs of the broken invocation with and without
     a particular config variable (and indeed, both cases produced no
     output!).

Let's fix the option, but also tweak the helper function to check the
exit code.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>