t9300: remove unnecessary use of /dev/stdin

t9300: remove unnecessary use of /dev/stdin

We really shouldn't be using these funny /dev/* files that did not exist
in V7 UNIX in our tests when we do not have to.

Output from

    $ git grep -n -e /dev/ --and --not -e /dev/null t/

tells us that, aside from use of /dev/urandom in apache.conf used in http
tests, "dd if=/dev/stdin" added recently to t/t9300-fast-import.sh are the
only offenders, and "dd" reads from the standard input by default, so
removing them should be straightforward.

Reported-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>