Require positive jenkins vote for merge-check

The merge-check runs on every open change when a change merges
to a repo.  But if the change has a merge conflict, Zuul will
leave repeated -1 messages about the failure.

Add a pipeline requirement to the merge-check pipeline so that
it will only test changes that currently have a positive jenkins
result.

Change-Id: I54ad8ba41e3f7b18fcc483b0d29a5322b71add42