dispatch: don't change error status if flushing stdio fails

If we already have a non-zero exit code, I don't think we should
change it to 255 because we fail to flush stdio. This may not matter
yet, but it will matter when I make a killed pager result in exit code
250 (it's currently 255).

Differential Revision: https://phab.mercurial-scm.org/D11626

mercurial/dispatch.py

@@ -253,7 +253,7 b' def dispatch(req):'
         status = -1
 
     ret = _flushstdio(req.ui, err)
-    if ret:
+    if ret and not status:
         status = ret
     return status