Require changes be open in check/gate

Only enqueue changes in the check or gate queues if they are "open"
in Gerrit.  This is required to break a test-comment loop caused by
the 'freshness' requirement on old abandoned changes.

Change-Id: I5e3c10295a71477e91e115d98a49a305d7e33adf
Merge "Require changes be open in check/gate"