convert: move packet_write_line() into pkt-line as packet_writel()

Add packet_writel() which writes multiple lines in a single call and
then calls packet_flush_gently(). Update convert.c to use the new
packet_writel() function from pkt-line.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>