The upload-git-mirror job currently reports failure when multiple
jobs run concurrently for the same repository (one of them succeeds,
the other fails because the state of the mirror was changed by the
successful one).
However, since the job generally runs a few minutes after the commit
that triggered it merged, the successful job usually mirrored the two
needed commits, so the failed mirror operation does not mean the
mirror is out-of-sync. This results in false negatives.
If zuul.newrev is provided, rather than report failure if the mirror
operation fails, we should report failure if, after the mirror operation,
the commit that triggered the job is still not present on the mirror.
Change-Id: Iadae26473f4c937384c8cea74329e389c563aeed
e53fe850c8