Add shortcut for checking ybd branches.

Branches in definitions that follow the pattern "staging/fromybd/$1" will git checkout ybd -b $1

Here is an example on a live merge request: https://gitlab.com/baserock/definitions/pipelines/5328228#build

Here is an example of it failing because the corresponding ybd branch does not exist: https://gitlab.com/baserock/definitions/builds/7222720