po: add '--no-wrap' to msgmerge, to not wrap long message lines

This avoids unneeded churn during POT-file renewal and PO-file updates,
as the PO files at the TP are made with '--no-wrap'.

Signed-off-by: Benno Schulenberg <bensberg@justemail.net>
Acked-by: Sami Kerola <kerolasa@iki.fi>