pkt-line: support tracing verbatim pack contents

pkt-line: support tracing verbatim pack contents

When debugging the pack protocol, it is sometimes useful to
store the verbatim pack that we sent or received on the
wire. Looking at the on-disk result is often not helpful for
a few reasons:

  1. If the operation is a clone, we destroy the repo on
     failure, leaving nothing on disk.

  2. If the pack is small, we unpack it immediately, and the
     full pack never hits the disk.

  3. If we feed the pack to "index-pack --fix-thin", the
     resulting pack has the extra delta bases added to it.

We already have a GIT_TRACE_PACKET mechanism for tracing
packets. Let's extend it with GIT_TRACE_PACKFILE to dump the
verbatim packfile.

There are a few other positive fallouts that come from
rearranging this code:

 - We currently disable the packet trace after seeing the
   PACK header, even though we may get human-readable lines
   on other sidebands; now we include them in the trace.

 - We currently try to print "PACK ..." in the trace to
   indicate that the packfile has started. But because we
   disable packet tracing, we never printed this line. We
   will now do so.

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