pkt-line: eliminate the need for static buffer in packet_write_gently()

pkt-line: eliminate the need for static buffer in packet_write_gently()

Teach `packet_write_gently()` to write the pkt-line header and the actual
buffer in 2 separate calls to `write_in_full()` and avoid the need for a
static buffer, thread-safe scratch space, or an excessively large stack
buffer.

Change `write_packetized_from_fd()` to allocate a temporary buffer rather
than using a static buffer to avoid similar issues here.

These changes are intended to make it easier to use pkt-line routines in
a multi-threaded context with multiple concurrent writers writing to
different streams.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>