Reviewed by Eric Seidel.

        commit-queue is slow during the day
        https://bugs.webkit.org/show_bug.cgi?id=45780

        Thanks to the new logging, we've noticed that checkout-is-out-of-date
        errors in the first pass of landing don't retry the land.  Instead,
        they're treated as failures and cause the commit-queue to do two more
        builds before really trying to land the patch.  Worse, in the second
        build, we can get bitten by a flaky test.

        This patch takes a slightly different approach to the commit-queue's
        main control logic.  We now use a separate subprocess for building and
        testing and for landing.  This means we should very rarely see the
        checkout-is-out-of-date error, and when we do see it, we should retry
        more quickly.  If my understanding is correct, this should be a big
        speed win for the commit-queue.

        * Scripts/webkitpy/tool/commands/download.py:
        * Scripts/webkitpy/tool/commands/queues.py:
        * Scripts/webkitpy/tool/commands/queues_unittest.py:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@67495 268f45cc-cd09-0410-ab3c-d52691b4dbfc