Reimplement current_branch with Rugged
Reimplement open with Rugged
Reimplement discard in Rugged
Remove shell from Repo init
Replace list with Rugged implementation
Reimplement nothing_to_commit? in Rugged
Eliminate include and include_from.

Two reasons justify this removal:

- Couldn't find good merge support in Rugged
- Merging is probably too complicated for "simplified git"
Move is_repo? to rugged
Switch init to use Rugged
Remove open3 dependency