IMPALA-1287: explain_level=0 for merging exchange is too verbose

This patch reduces the verbosity of the of the explain string
for the merge exchange node by reducing the order information
for explain_level=0.

Change-Id: I13bd0889bbc18aa5b60ea0721e8d8b08cde57714
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4856
Reviewed-by: Martin Grund <mgrund@cloudera.com>
Tested-by: jenkins