Linear flow poorly handles empty flows in it.

[Yuriy Taraday]

When some empty flow is inserted into linear flow, dependencies of tasks that follow this empty flow are ignored.

Having this minimal script: http://paste.openstack.org/show/133084/ I got this exception with master taskflow: http://paste.openstack.org/show/133078/ and this with latest released version: http://paste.openstack.org/show/133085/

I both cases there's no edges in execution graph at all: http://paste.openstack.org/show/133087/

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/135146

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/135146
Committed: https://git.openstack.org/cgit/openstack/taskflow/commit/?id=eaf49950383865438dd5aaef05de7d86977403f5
Submitter: Jenkins
Branch: master

commit eaf49950383865438dd5aaef05de7d86977403f5
Author: Joshua Harlow <email address hidden>
Date: Mon Nov 17 16:28:49 2014 -0800

    Properly handle and skip empty intermediary flows

    Instead of linking nodes which have elements to
    predecessors which do not we should search backwards
    through the prior predecessors and link to one that
    does have nodes; this ensures that we do not create
    bad workflows when empty flows are injected.

    Fixes bug 1392650

    Change-Id: Ic362ef3400f9c77e60ed07b0097e3427b999d1cd