Logically separate main branch and base branch

[Silic0nS0ldier]

Take the following branching structure:

1. A green branch which contains snapshots of the primary merge target (from when its build passes)
2. The aforementioned primary merge target (master, main, dev, whatever)
3. A branch with new changes, based off green (to provide a known good base branch) but to be merged into the primary merge target.

Currently with PR train the merge target and base branch are assumed to be the same (in .pr-train.yml that is). It would be nice if another branch option could be specified (rebase-branch-name as an example).