Merge branch 'jk/push-client-deadlock-fix' into HEAD

Some Windows SDK lacks pthread_sigmask() implementation and fails
to compile the recently updated "git push" codepath that uses it.

* jk/push-client-deadlock-fix:
  Windows: only add a no-op pthread_sigmask() when needed
  Windows: add pthread_sigmask() that does nothing
  t5504: drop sigpipe=ok from push tests
  fetch-pack: isolate sigpipe in demuxer thread
  send-pack: isolate sigpipe in demuxer thread
  run-command: teach async threads to ignore SIGPIPE
  send-pack: close demux pipe before finishing async process

run-command.h

@@ -116,6 +116,7 @@ struct async {
 	int proc_in;
 	int proc_out;
 #endif
+	int isolate_sigpipe;
 };
 
 int start_async(struct async *async);