t0000: do not use export X=Y

The shell syntax "export X=Y A=B" is not understood by all shells.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Acked-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>