pkt-line: add functions to read/write flush terminated packet streams
write_packetized_from_fd() and write_packetized_from_buf() write a
stream of packets. All content packets use the maximal packet size
except for the last one. After the last content packet a `flush` control
packet is written.
read_packetized_to_strbuf() reads arbitrary sized packets until it
detects a `flush` packet.
Signed-off-by: Lars Schneider <larsxschneider@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>