sshpeer: don't fail forwarding output from closed connections

The test still shows an internal error, but one that happens
further along.

Differential Revision: https://phab.mercurial-scm.org/D10073

mercurial/sshpeer.py

@@ -40,7 +40,7 b' def _forwardoutput(ui, pipe, warn=False)'
     """display all data currently available on pipe as remote output.
 
     This is non blocking."""
-    if pipe:
+    if pipe and not pipe.closed:
         s = procutil.readpipe(pipe)
         if s:
             display = ui.warn if warn else ui.status

tests/test-ssh-batch.t

@@ -10,4 +10,4 b' fails (thus causing the sshpeer to be st'
 further lookups don't result in tracebacks.
 
   $ hg pull -r b0 -r nosuchbookmark $(for i in $($TESTDIR/seq.py 1 20); do echo -r b$i; done) -e "\"$PYTHON\" \"$TESTDIR/dummyssh\"" ssh://user@dummy/$(pwd)/../a |& tail -n 1
-  ValueError: I/O operation on closed file
+  StopIteration