git prune-branches

@kevgo please review

+ Changed a couple git helpers to take in parameters instead of operating on the current branch
+ Set the default branch on the remote repo to main (needed as the git prune-branches feature specs were deleting the master branch (since its not main) and you cannot delete the HEAD branch on the remote repo.
If I mistype "behind", for example as "behid", it creates a branch ahead of main. It is more robust to check all possible values here, and fail with a meaningful error message for unknown values.

```ruby
commit_to_branch = case relation
  when 'behind' then 'main'
  when 'ahead of' then branch_name
  else raise "unknown relation: '#{relation}'"
end
```