PackWriter: Display totals after sending objects

PackWriter: Display totals after sending objects

CGit pack-objects displays a totals line after the pack data
was fully written.  This can be useful to understand some of
the decisions made by the packer, and has been a great tool
for helping to debug some of that code.

Track some of the basic values, and send it to the client when
packing is done:

  remote: Counting objects: 1826776, done
  remote: Finding sources: 100% (55121/55121)
  remote: Getting sizes: 100% (25654/25654)
  remote: Compressing objects: 100% (11434/11434)
  remote: Total 1861830 (delta 3926), reused 1854705 (delta 38306)
  Receiving objects: 100% (1861830/1861830), 386.03 MiB | 30.32 MiB/s, done.

Change-Id: If3b039017a984ed5d5ae80940ce32bda93652df5
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>