In the previous data model the build pipelines were nested inside
the pipeline and thus we would recurse in `build_stages`. The
tree that was built was returned and potentially became the build
tree for the pipeline that invoked `build_stages`. In the new
model of a direct acyclic graph of pipelines the build tree can
be any previously built pipeline and we just get it via the store,
which now keeps track of all previously built pipelines even if
there are not committed to it. Thus there is no need to return
the trees from `build_stages` anymore.
Adjust the short code that does the short circuit check to use
`ObjectStore.contains` instead of `ObjectStore.get` since we
do not need to object anymore.
bd28a29b85