Teach upload-pack to log the received need lines to an fd

Teach upload-pack to log the received need lines to an fd

To facilitate testing and verification of the requests sent by
git-fetch to the remote side we permit logging the received packet
lines to the file descriptor specified in GIT_DEBUG_SEND_PACK has
been set.  Special start and end lines are included to indicate
the start and end of each connection.

  $ GIT_DEBUG_SEND_PACK=3 git fetch 3>UPLOAD_LOG
  $ cat UPLOAD_LOG
  #S
  want 8e10cf4e007ad7e003463c30c34b1050b039db78 multi_ack side-band-64k thin-pack ofs-delta
  want ddfa4a33562179aca1ace2bcc662244a17d0b503
  #E
  #S
  want 3253df4d1cf6fb138b52b1938473bcfec1483223 multi_ack side-band-64k thin-pack ofs-delta
  #E

>From the above trace the first connection opened by git-fetch was to
download two refs (with values 8e and dd) and the second connection
was opened to automatically follow an annotated tag (32).

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>