t0008: use named pipe (FIFO) to test check-ignore streaming

sleeps in the check-ignore test suite are not ideal since they can
fail when the system is under load, or when a tool like valgrind is
used which drastically alters the timing.  Therefore we replace them
with a more robust solution using a named pipe (FIFO).

Thanks to Jeff King for coming up with the redirection wizardry
required to make this work.

http://article.gmane.org/gmane.comp.version-control.git/220916

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'as/check-ignore' into next

Update its tests to avoid race.

* as/check-ignore:
  t0008: use named pipe (FIFO) to test check-ignore streaming