stage: require start point and branch

[olets]

Currently you can config

stage:
  start-point:
    - my-branch

The result will be

% git replay
Resetting 0 to master
git checkout -B 0 master
Switched to a new branch '0'
Replay complete
% git branch --show-current 
0

Enforce the necessary depth. The above configuration should give an error (based on git-replay's existing error messages). Should have to be something like

stage:
  start-point:
    branch: commit