Remove problematic go-git usage

Commits since tag 0.3.0

1953c54 DRY up code around git commands
3b7a53a Remove unused code
7370c50 Remove repo.Reference use
1e5d624 Land shell
7d5acb6 Update Bazel & release build
d9ab844 Attach stdin/stdout/stderr to git commit
f16c02a Add diff command to arc
305829d Start of arc diff
51c8761 Fix help formatting
59befa3 https
2641d53 Add release info
d3241f7 Add Bazel and release script
67b9713 Cleanup README