scripts: "export VAR=VALUE" construct is not portable

Found by check-non-portable-shell.pl

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
scripts: more "export VAR=VALUE" fixes

Found by

    git grep '[^-]export [^&]*=' -- \*.sh

Signed-off-by: Junio C Hamano <gitster@pobox.com>