pkt-line: do not issue flush packets in write_packetized_*()

pkt-line: do not issue flush packets in write_packetized_*()

Remove the `packet_flush_gently()` call in `write_packetized_from_buf() and
`write_packetized_from_fd()` and require the caller to call it if desired.
Rename both functions to `write_packetized_from_*_no_flush()` to prevent
later merge accidents.

`write_packetized_from_buf()` currently only has one caller:
`apply_multi_file_filter()` in `convert.c`.  It always wants a flush packet
to be written after writing the payload.

However, we are about to introduce a caller that wants to write many
packets before a final flush packet, so let's make the caller responsible
for emitting the flush packet.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>