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
c555e529ac