Use shorter version of base_ref for branches in the canonical repos…

…itory

This fixes a regression caused by mozilla#276 that has caused decision tasks to fail for non `main` branches in this repository.

The "fix" is to refer to these branches by their short name (rather than `refs/heads/foo`). (This is not a great fix...but it's what we have to do to work around taskcluster/taskgraph#380 for now.)