P4 on Windows expects the PWD environment variable to be set to the
current working dir, but os.chdir in python doesn't do so.
Signed-off-by: Robert Blum <rob.blum@gmail.com> Acked-by: Simon Hausmann <simon@lst.de> Acked-by: Han-Wen Nienhuys <hanwen@xs4all.nl> Signed-off-by: Junio C Hamano <gitster@pobox.com>