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

compat/mingw.h

@@ -142,6 +142,7 @@ static inline int fcntl(int fd, int cmd, ...)
 #define sigemptyset(x) (void)0
 static inline int sigaddset(sigset_t *set, int signum)
 { return 0; }
+#define SIG_BLOCK 0
 #define SIG_UNBLOCK 0
 static inline int sigprocmask(int how, const sigset_t *set, sigset_t *oldset)
 { return 0; }