Parse commit messages for "Depends-On: <changeid>" and treat
matching changes as changes that the given change depends on.
This will treat any changes in any branch of any project as
such. If the projects share a dependent change queue, the
changes will be enqueued in order. If they do not share a
change queue in a dependent pipeline, then the latter one will
be unable to be enqueued until the change it depends on merges.
If the dependencies result in a cycle, Zuul will log the error
but otherwise the problematic changes will be ignored.
Dependent changes in independent pipelines are not yet addressed.
Change-Id: I90c173f86d11e6c44d1f408646589b7c75b1cd52
5ee2425651