pkt-line: show packets in async processes as "sideband"

pkt-line: show packets in async processes as "sideband"

If you run "GIT_TRACE_PACKET=1 git push", you may get
confusing output like (line prefixes omitted for clarity):

   packet:      push< \1000eunpack ok0019ok refs/heads/master0000
   packet:      push< unpack ok
   packet:      push< ok refs/heads/master
   packet:      push< 0000
   packet:      push< 0000

Why do we see the data twice, once apparently wrapped inside
another pkt-line, and once unwrapped? Why do we get two
flush packets?

The answer is that we start an async process to demux the
sideband data. The first entry comes from the sideband
process reading the data, and the second from push itself.
Likewise, the first flush is inside the demuxed packet, and
the second is an actual sideband flush.

We can make this a bit more clear by marking the sideband
demuxer explicitly as "sideband" rather than "push". The
most elegant way to do this would be to simply call
packet_trace_identity() inside the sideband demuxer. But we
can't do that reliably, because it relies on a global
variable, which might be shared if pthreads are in use.

What we really need is thread-local storage for
packet_trace_identity. But the async code does not provide
an interface for that, and it would be messy to add it here
(we'd have to care about pthreads, initializing our
pthread_key_t ahead of time, etc).

So instead, let us just assume that any async process is
handling sideband data. That's always true now, and is
likely to remain so in the future.

The output looks like:

   packet:  sideband< \1000eunpack ok0019ok refs/heads/master0000
   packet:      push< unpack ok
   packet:      push< ok refs/heads/master
   packet:      push< 0000
   packet:  sideband< 0000

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>