Fixes to clone process

- No longer create or depend on a "master" branch
- Bail if a rerere cache already exists so we don't
  mess something up
- Only track the rr-cache branch for the rr-cache (crazy idea!)
Add bmpt checkout, alter behavior of start

- If start is called on a story without a branch,
  a branch is created and the story is started.
- If start is called on a story with a branch,
  that branch is checked out and the story is restarted.
- If start is called without any arguments,
  the story of the current branch is restarted.