commandserver: unblock SIGCHLD

This enables the SIGCHLD handler to work properly if some buggy program
started chg server with SIGCHLD blocked.

A test of this probably requires C code, but we don't have such kind of
tests already. Since this is a simple and clear fix, I'm leaving it as
"untested" but I did a manual test and there were no longer zombie workers.

Differential Revision: https://phab.mercurial-scm.org/D1737

mercurial/commandserver.py

@@ -449,6 +449,8 b' class unixforkingservice(object):'
     def init(self):
         self._sock = socket.socket(socket.AF_UNIX)
         self._servicehandler.bindsocket(self._sock, self.address)
+        if util.safehasattr(util, 'unblocksignal'):
+            util.unblocksignal(signal.SIGCHLD)
         o = signal.signal(signal.SIGCHLD, self._sigchldhandler)
         self._oldsigchldhandler = o
         self._socketunlinked = False