remote-curl: Fix push status report when all branches fail

remote-curl: Fix push status report when all branches fail

The protocol between transport-helper.c and remote-curl requires
remote-curl to always print a blank line after the push command
has run. If the blank line is ommitted, transport-helper kills its
container process (the git push the user started) with exit(128)
and no message indicating a problem, assuming the helper already
printed reasonable error text to the console.

However if the remote rejects all branches with "ng" commands in the
report-status reply, send-pack terminates with non-zero status, and
in turn remote-curl exited with non-zero status before outputting
the blank line after the helper status printed by send-pack. No
error messages reach the user.

This caused users to see the following from git push over HTTP
when the remote side's update hook rejected the branch:

  $ git push http://... master
  Counting objects: 4, done.
  Delta compression using up to 6 threads.
  Compressing objects: 100% (2/2), done.
  Writing objects: 100% (3/3), 301 bytes, done.
  Total 3 (delta 0), reused 0 (delta 0)
  $

Always print a blank line after the send-pack process terminates,
ensuring the helper status report (if it was output) will be
correctly parsed by the calling transport-helper.c. This ensures
the helper doesn't abort before the status report can be shown to
the user.

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