t5504: handle expected output from SIGPIPE death

t5504: handle expected output from SIGPIPE death

Commit 8bf4bec (add "ok=sigpipe" to test_must_fail and use
it to fix flaky tests, 2015-11-27) taught t5504 to handle
"git push" racily exiting with SIGPIPE rather than failing.

However, one of the tests checks the output of the command,
as well. In the SIGPIPE case, we will not have produced any
output. If we want the test to be truly non-flaky, we have
to accept either output.

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