"git push" had a handful of codepaths that could lead to a deadlock
when unexpected error happened, which has been fixed.

* jk/push-deadlock-regression-fix:
  send-pack: report signal death of pack-objects
  send-pack: read "unpack" status even on pack-objects failure
  send-pack: improve unpack-status error messages
  send-pack: use skip_prefix for parsing unpack status
  send-pack: extract parsing of "unpack" response
  receive-pack: fix deadlock when we cannot create tmpdir