cmdserver: don't raise EOFError when trying to read 0 bytes from the client

mercurial/commandserver.py

@@ -155,6 +155,9 b' class server(object):'
             raise util.Abort(_('unknown mode %s') % mode)
 
     def _read(self, size):
+        if not size:
+            return ''
+
         data = self.client.read(size)
 
         # is the other end closed?