[PATCH 0/5] fix deadlock in git-push

I ran across a deadlock today while pushing from a corrupted repository
where pack-objects fails. Obviously I don't expect this to succeed, but
it should not hang indefinitely.

The first patch below fixes the deadlock. Unfortunately, it turns it
into a likely SIGPIPE death. Which is an improvement, but not ideal.

Patches 2 and 3 address that by fixing the way we handle SIGPIPE in
async threads.

Patches 4 and 5 are cleanups to earlier topics that are enabled by the
new SIGPIPE handling.

  [1/5]: send-pack: close demux pipe before finishing async process
  [2/5]: run-command: teach async threads to ignore SIGPIPE
  [3/5]: send-pack: isolate sigpipe in demuxer thread
  [4/5]: fetch-pack: isolate sigpipe in demuxer thread
  [5/5]: t5504: drop sigpipe=ok from push tests

I confirmed that after this series we still pass the stress test I
outlined in

  http://article.gmane.org/gmane.comp.version-control.git/287176

So hopefully I haven't made anything worse (and that the tightening in
5/5 isn't just bringing back some test flakiness).

-Peff