So that we may re-use the same jobs for pre and post merge tests,
enqueue an item for every branch of every timer-triggered project
and checkout that branch before running the job. This means that
rather than having a job for gate plus a job for each stable branch,
we hav just have a single job which runs with different content.
The old method is still supported using override branches.
This updates the model to include Change, Branch, Tag, and Ref
objects which can be used as the value of Item.change. Branch,
Tag, and Ref are all very similar, but the distinction may help
us ensure that we're encoding the right information about the items
we are enqueing. This is important for branch matching in pipelines
and is also used to provide job variables.
Change-Id: I5c41d2dcbbbd1c17d68074cd7480e6ab83f884ea
2103778ae4