synchroniser : use -S to avoid login in

We create a master ssh connection the -S flag uses this to avoid having
to log in again.

src/unisync/synchroniser.py

@@ -217,6 +217,7 @@ class Synchroniser:
             command.append(value)
 
         sshargs = f"-p {self.remote_port} "
+        sshargs += f"-S {self.control_path} "
         for arg, value in self.ssh_settings.items():
             sshargs += arg + " " + value + " "
         command.append("-sshargs")