STM: fix for PG and "Proof term" lines.

PG sends "Set Silent" and it was messing up the DAG, making the
detection of an immediate proof not working.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@17061 85f007b7-540e-0410-9357-904b9bb8a0f7