t1006: ensure cat-file info isn't buffered by default

t1006: ensure cat-file info isn't buffered by default

While working on buffering changes to `git cat-file' in a
separate patch, I inadvertently made the output of --batch-check
and the `info' command of --batch-command buffered as if
opt->buffer_output is turned on by default.

Buffering by default breaks some 3rd-party Perl scripts using
cat-file, but this breakage was not detected anywhere in our
test suite.  Add a small Perl snippet to test this problem since
(AFAIK) other equivalent ways to test this behavior from Bourne
shell and/or awk would require racy sleeps, non-portable FIFOs
or tedious C code.

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>