v3, 8 partsOptimized tag autofollowing in git-fetch

Here's version 3 of the more aggressive tag following for git-fetch.

  1)  Remove unused variable in builtin-fetch find_non_local_tags
  2)  Remove unnecessary delaying of free_refs(ref_map) in builtin-fetch
  3)  Ensure tail pointer gets setup correctly when we fetch HEAD only
  4)  Allow builtin-fetch's find_non_local_tags to append onto a list
  5)  Free the path_lists used to find non-local tags in git-fetch
  6)  Teach upload-pack to log the received need lines to an fd
  7)  Make git-fetch follow tags we already have objects for sooner
  8)  Teach git-fetch to grab a tag at the same time as a commit

 builtin-fetch.c      |   49 +++++++++++++------
 t/t5503-tagfollow.sh |  124 ++++++++++++++++++++++++++++++++++++++++++++++++++
 upload-pack.c        |    9 ++++
 3 files changed, 166 insertions(+), 16 deletions(-)

Changes since the last (v2) round:

  "Free the path_lists used to find non-local tags in git-fetch"

    This patch was added to the series to avoid memory corruption
    errors that were only showing up on Linux.  My earlier testing
    only on Mac OS X failed to show any symptoms of the corruption.
    Junio noticed it on Linux and asked me to revisit the series.

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

    At Daniel's suggestion the GIT_DEBUG_SEND_PACK environment
    variable takes the value of the fd we log onto.  This mirrors
    how GIT_TRACE works.