t7700: annotate cruft-pack failure with ok=sigpipe

t7700: annotate cruft-pack failure with ok=sigpipe

One of our tests intentionally causes the cruft-pack generation phase of
repack to fail, in order to stimulate an exit from repack at the desired
moment. It does so by feeding a bogus option argument to pack-objects.
This is a simple and reliable way to get pack-objects to fail, but it
has one downside: pack-objects will die before reading its stdin, which
means the caller repack may racily get SIGPIPE writing to it.

For the purposes of this test, that's OK. We are checking whether repack
cleans up already-created .tmp files, and it will do so whether it exits
or dies by signal (because the tempfile API hooks both).

But we have to tell test_must_fail that either outcome is OK, or it
complains about the signal. Arguably this is a workaround (compared to
fixing repack), as repack dying to SIGPIPE means that it loses the
opportunity to give a more detailed message. But we don't actually write
such a message anyway; we rely on pack-objects to have written something
useful to stderr, and it does. In either case (signal or exit), that is
the main thing the user will see.

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