Automatically detect branches that have changed to run OBT on

At the moment the branches that the OBT runs are manually defined and need adjusting if another upstream branch is worked on.

As an option, it would be useful if OBT could automatically run on branch that had changed.
So that it automatically picked up new candidate branches and didn't rerun on branches that were no longer changing.

It might also be useful to continue to run branches for a N days since last changed, so that even when it hadn't changed there were a few OBT runs done, that might flush out any inconsistencies, etc.