Improve the interactive UI

[igloo]

Currently the interactive patch selections is only used by record, and it is very primitive. It needs to be filled out into a full solution.

However, we need to be careful not to use memory unnecessarily (e.g. darcs record -a and darc record; a should have similar memory usage) or read patches unnecessarily (e.g. unpull shouldn't read all the patches in order to unpull the most recent).