filter-branch: prevent filters from reading from stdin

filter-branch: prevent filters from reading from stdin

stdin is the list of commits when the env, tree and index
filter are executed.  The filters are not supposed to read
anything from stdin so the best is to give them /dev/null
for reading.

Signed-off-by: Matthias Lederhofer <matled@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>