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

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

--- a/mercurial/commandserver.py	Mon Jun 20 19:08:41 2011 -0300
+++ b/mercurial/commandserver.py	Tue Jun 21 15:13:39 2011 +0300
@@ -155,6 +155,9 @@
             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?